diff --git a/cryptography/hazmat/backends/interfaces.py b/cryptography/hazmat/backends/interfaces.py
index 20c2111..e4c1df3 100644
--- a/cryptography/hazmat/backends/interfaces.py
+++ b/cryptography/hazmat/backends/interfaces.py
@@ -18,7 +18,8 @@
 import six
 
 
-class CipherBackend(six.with_metaclass(abc.ABCMeta)):
+@six.add_metaclass(abc.ABCMeta)
+class CipherBackend(object):
     @abc.abstractmethod
     def cipher_supported(self, cipher, mode):
         """
@@ -38,7 +39,8 @@
         """
 
 
-class HashBackend(six.with_metaclass(abc.ABCMeta)):
+@six.add_metaclass(abc.ABCMeta)
+class HashBackend(object):
     @abc.abstractmethod
     def hash_supported(self, algorithm):
         """
@@ -52,7 +54,8 @@
         """
 
 
-class HMACBackend(six.with_metaclass(abc.ABCMeta)):
+@six.add_metaclass(abc.ABCMeta)
+class HMACBackend(object):
     @abc.abstractmethod
     def hmac_supported(self, algorithm):
         """
@@ -67,7 +70,8 @@
         """
 
 
-class PBKDF2HMACBackend(six.with_metaclass(abc.ABCMeta)):
+@six.add_metaclass(abc.ABCMeta)
+class PBKDF2HMACBackend(object):
     @abc.abstractmethod
     def pbkdf2_hmac_supported(self, algorithm):
         """
@@ -83,7 +87,8 @@
         """
 
 
-class RSABackend(six.with_metaclass(abc.ABCMeta)):
+@six.add_metaclass(abc.ABCMeta)
+class RSABackend(object):
     @abc.abstractmethod
     def generate_rsa_private_key(self, public_exponent, key_size):
         """
@@ -113,7 +118,8 @@
         """
 
 
-class DSABackend(six.with_metaclass(abc.ABCMeta)):
+@six.add_metaclass(abc.ABCMeta)
+class DSABackend(object):
     @abc.abstractmethod
     def generate_dsa_parameters(self, key_size):
         """
@@ -128,7 +134,8 @@
         """
 
 
-class OpenSSLSerializationBackend(six.with_metaclass(abc.ABCMeta)):
+@six.add_metaclass(abc.ABCMeta)
+class OpenSSLSerializationBackend(object):
     @abc.abstractmethod
     def load_openssl_pem_private_key(self, data, password):
         """
diff --git a/cryptography/hazmat/bindings/openssl/bignum.py b/cryptography/hazmat/bindings/openssl/bignum.py
index d505b17..94c2914 100644
--- a/cryptography/hazmat/bindings/openssl/bignum.py
+++ b/cryptography/hazmat/bindings/openssl/bignum.py
@@ -68,6 +68,7 @@
 
 int BN_num_bits(const BIGNUM *);
 
+int BN_cmp(const BIGNUM *, const BIGNUM *);
 int BN_add(BIGNUM *, const BIGNUM *, const BIGNUM *);
 int BN_sub(BIGNUM *, const BIGNUM *, const BIGNUM *);
 int BN_mul(BIGNUM *, const BIGNUM *, const BIGNUM *, BN_CTX *);
@@ -92,7 +93,6 @@
 int BN_zero(BIGNUM *);
 int BN_one(BIGNUM *);
 int BN_mod(BIGNUM *, const BIGNUM *, const BIGNUM *, BN_CTX *);
-BIGNUM *BN_cmp(const BIGNUM *, const BIGNUM *);
 """
 
 CUSTOMIZATIONS = """
diff --git a/cryptography/hazmat/primitives/interfaces.py b/cryptography/hazmat/primitives/interfaces.py
index eab48b4..e70338b 100644
--- a/cryptography/hazmat/primitives/interfaces.py
+++ b/cryptography/hazmat/primitives/interfaces.py
@@ -18,7 +18,8 @@
 import six
 
 
-class CipherAlgorithm(six.with_metaclass(abc.ABCMeta)):
+@six.add_metaclass(abc.ABCMeta)
+class CipherAlgorithm(object):
     @abc.abstractproperty
     def name(self):
         """
@@ -32,7 +33,8 @@
         """
 
 
-class BlockCipherAlgorithm(six.with_metaclass(abc.ABCMeta)):
+@six.add_metaclass(abc.ABCMeta)
+class BlockCipherAlgorithm(object):
     @abc.abstractproperty
     def block_size(self):
         """
@@ -40,7 +42,8 @@
         """
 
 
-class Mode(six.with_metaclass(abc.ABCMeta)):
+@six.add_metaclass(abc.ABCMeta)
+class Mode(object):
     @abc.abstractproperty
     def name(self):
         """
@@ -55,7 +58,8 @@
         """
 
 
-class ModeWithInitializationVector(six.with_metaclass(abc.ABCMeta)):
+@six.add_metaclass(abc.ABCMeta)
+class ModeWithInitializationVector(object):
     @abc.abstractproperty
     def initialization_vector(self):
         """
@@ -63,7 +67,8 @@
         """
 
 
-class ModeWithNonce(six.with_metaclass(abc.ABCMeta)):
+@six.add_metaclass(abc.ABCMeta)
+class ModeWithNonce(object):
     @abc.abstractproperty
     def nonce(self):
         """
@@ -71,7 +76,8 @@
         """
 
 
-class ModeWithAuthenticationTag(six.with_metaclass(abc.ABCMeta)):
+@six.add_metaclass(abc.ABCMeta)
+class ModeWithAuthenticationTag(object):
     @abc.abstractproperty
     def tag(self):
         """
@@ -79,7 +85,8 @@
         """
 
 
-class CipherContext(six.with_metaclass(abc.ABCMeta)):
+@six.add_metaclass(abc.ABCMeta)
+class CipherContext(object):
     @abc.abstractmethod
     def update(self, data):
         """
@@ -94,7 +101,8 @@
         """
 
 
-class AEADCipherContext(six.with_metaclass(abc.ABCMeta)):
+@six.add_metaclass(abc.ABCMeta)
+class AEADCipherContext(object):
     @abc.abstractmethod
     def authenticate_additional_data(self, data):
         """
@@ -102,7 +110,8 @@
         """
 
 
-class AEADEncryptionContext(six.with_metaclass(abc.ABCMeta)):
+@six.add_metaclass(abc.ABCMeta)
+class AEADEncryptionContext(object):
     @abc.abstractproperty
     def tag(self):
         """
@@ -111,7 +120,8 @@
         """
 
 
-class PaddingContext(six.with_metaclass(abc.ABCMeta)):
+@six.add_metaclass(abc.ABCMeta)
+class PaddingContext(object):
     @abc.abstractmethod
     def update(self, data):
         """
@@ -125,7 +135,8 @@
         """
 
 
-class HashAlgorithm(six.with_metaclass(abc.ABCMeta)):
+@six.add_metaclass(abc.ABCMeta)
+class HashAlgorithm(object):
     @abc.abstractproperty
     def name(self):
         """
@@ -145,7 +156,8 @@
         """
 
 
-class HashContext(six.with_metaclass(abc.ABCMeta)):
+@six.add_metaclass(abc.ABCMeta)
+class HashContext(object):
     @abc.abstractproperty
     def algorithm(self):
         """
@@ -171,7 +183,8 @@
         """
 
 
-class RSAPrivateKey(six.with_metaclass(abc.ABCMeta)):
+@six.add_metaclass(abc.ABCMeta)
+class RSAPrivateKey(object):
     @abc.abstractproperty
     def modulus(self):
         """
@@ -255,7 +268,8 @@
         """
 
 
-class RSAPublicKey(six.with_metaclass(abc.ABCMeta)):
+@six.add_metaclass(abc.ABCMeta)
+class RSAPublicKey(object):
     @abc.abstractproperty
     def modulus(self):
         """
@@ -287,7 +301,8 @@
         """
 
 
-class DSAParameters(six.with_metaclass(abc.ABCMeta)):
+@six.add_metaclass(abc.ABCMeta)
+class DSAParameters(object):
     @abc.abstractproperty
     def modulus(self):
         """
@@ -333,7 +348,8 @@
         """
 
 
-class DSAPrivateKey(six.with_metaclass(abc.ABCMeta)):
+@six.add_metaclass(abc.ABCMeta)
+class DSAPrivateKey(object):
     @abc.abstractproperty
     def key_size(self):
         """
@@ -365,7 +381,8 @@
         """
 
 
-class DSAPublicKey(six.with_metaclass(abc.ABCMeta)):
+@six.add_metaclass(abc.ABCMeta)
+class DSAPublicKey(object):
     @abc.abstractproperty
     def key_size(self):
         """
@@ -385,7 +402,8 @@
         """
 
 
-class AsymmetricSignatureContext(six.with_metaclass(abc.ABCMeta)):
+@six.add_metaclass(abc.ABCMeta)
+class AsymmetricSignatureContext(object):
     @abc.abstractmethod
     def update(self, data):
         """
@@ -399,7 +417,8 @@
         """
 
 
-class AsymmetricVerificationContext(six.with_metaclass(abc.ABCMeta)):
+@six.add_metaclass(abc.ABCMeta)
+class AsymmetricVerificationContext(object):
     @abc.abstractmethod
     def update(self, data):
         """
@@ -414,7 +433,8 @@
         """
 
 
-class AsymmetricPadding(six.with_metaclass(abc.ABCMeta)):
+@six.add_metaclass(abc.ABCMeta)
+class AsymmetricPadding(object):
     @abc.abstractproperty
     def name(self):
         """
@@ -422,7 +442,8 @@
         """
 
 
-class KeyDerivationFunction(six.with_metaclass(abc.ABCMeta)):
+@six.add_metaclass(abc.ABCMeta)
+class KeyDerivationFunction(object):
     @abc.abstractmethod
     def derive(self, key_material):
         """
diff --git a/docs/hazmat/primitives/asymmetric/dsa.rst b/docs/hazmat/primitives/asymmetric/dsa.rst
index 1a6a6e0..2819bbd 100644
--- a/docs/hazmat/primitives/asymmetric/dsa.rst
+++ b/docs/hazmat/primitives/asymmetric/dsa.rst
@@ -37,13 +37,13 @@
 
         Generate a new ``DSAParameters`` instance using ``backend``.
 
-        :param int key_size: The length of the modulus in bits. It should be 
-            either "1024, 2048 or 3072". For keys generated in 2014 this should 
+        :param int key_size: The length of the modulus in bits. It should be
+            either "1024, 2048 or 3072". For keys generated in 2014 this should
             be `at least 2048`_ (See page 41).
-            Note that some applications (such as SSH) have not yet gained support 
+            Note that some applications (such as SSH) have not yet gained support
             for larger key sizes specified in FIPS 186-3 and are still restricted
             to only the 1024-bit keys specified in FIPS 186-2.
-            
+
         :return: A new instance of ``DSAParameters``
 
         :raises cryptography.exceptions.UnsupportedAlgorithm: This is raised if
@@ -92,7 +92,7 @@
         :raises cryptography.exceptions.UnsupportedAlgorithm: This is raised if
             the provided ``backend`` does not implement
             :class:`~cryptography.hazmat.backends.interfaces.DSABackend`
-        
+
         :raises ValueError: This is raised if the key size is not (1024 or 2048 or 3072)
             or if the OpenSSL version is older than 1.0.0 and the key size is larger than 1024
             because older OpenSSL versions don't support a key size larger than 1024.
@@ -115,11 +115,11 @@
     :raises TypeError: This is raised when the arguments are not all integers.
 
     :raises ValueError: This is raised when the values of ``modulus``,
-                        ``subgroup_order``,``generator``, or ``y`` 
+                        ``subgroup_order``, ``generator``, or ``y``
                         do not match the bounds specified in `FIPS 186-4`_.
 
 
-.. _`DSA`: https://en.wikipedia.org/wiki/Digital_Signature_Algorithm 
+.. _`DSA`: https://en.wikipedia.org/wiki/Digital_Signature_Algorithm
 .. _`public-key`: https://en.wikipedia.org/wiki/Public-key_cryptography
-.. _`FIPS 186-4`: http://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.186-4.pdf 
+.. _`FIPS 186-4`: http://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.186-4.pdf
 .. _`at least 2048`: http://www.ecrypt.eu.org/documents/D.SPA.20.pdf
diff --git a/docs/installation.rst b/docs/installation.rst
index c6a2a5c..ac4c13c 100644
--- a/docs/installation.rst
+++ b/docs/installation.rst
@@ -94,19 +94,22 @@
 ------------------------------
 
 To link cryptography against a custom version of OpenSSL you'll need to set
-``ARCHFLAGS``, ``LDFLAGS``, and ``CFLAGS``. OpenSSL can be installed via
-`Homebrew`_:
+``ARCHFLAGS``, ``LDFLAGS``, and ``CFLAGS``. OpenSSL can be installed via `Homebrew`_ or `MacPorts`_:
+
+`Homebrew`_
 
 .. code-block:: console
 
     $ brew install openssl
+    $ env ARCHFLAGS="-arch x86_64" LDFLAGS="-L/usr/local/opt/openssl/lib" CFLAGS="-I/usr/local/opt/openssl/include" pip install cryptography
 
-Then install cryptography linking against the brewed version:
+or `MacPorts`_:
 
 .. code-block:: console
 
-    $ env ARCHFLAGS="-arch x86_64" LDFLAGS="-L/usr/local/opt/openssl/lib" CFLAGS="-I/usr/local/opt/openssl/include" pip install cryptography
-
+    $ sudo port install openssl
+    $ env ARCHFLAGS="-arch x86_64" LDFLAGS="-L/opt/local/lib" CFLAGS="-I/opt/local/include" pip install cryptography
 
 .. _`Homebrew`: http://brew.sh
+.. _`MacPorts`: http://www.macports.org
 .. _`pre-compiled binaries`: https://www.openssl.org/related/binaries.html