Scrypt Implementation (#3117)

* Scrypt implementation.

* Docs stuff.

* Make example just an example and not a doctest.

* Add changelog entry.

* Docs cleanup.

* Add more tests.

* Add multibackend tests.

* PEP8.

* Add docs about Scrypt parameters.

* Docs cleanup.

* Add AlreadyFinalized.
diff --git a/docs/hazmat/backends/openssl.rst b/docs/hazmat/backends/openssl.rst
index 8bc7dac..791aab3 100644
--- a/docs/hazmat/backends/openssl.rst
+++ b/docs/hazmat/backends/openssl.rst
@@ -24,6 +24,11 @@
     * :class:`~cryptography.hazmat.backends.interfaces.PEMSerializationBackend`
     * :class:`~cryptography.hazmat.backends.interfaces.X509Backend`
 
+    It also implements the following interface for OpenSSL versions ``1.1.0``
+    and above.
+
+    * :class:`~cryptography.hazmat.backends.interfaces.ScryptBackend`
+
     It also exposes the following:
 
     .. attribute:: name