Make the code slightly resillient to python4 (#880)

diff --git a/src/OpenSSL/crypto.py b/src/OpenSSL/crypto.py
index 5a734cc..b51e12f 100644
--- a/src/OpenSSL/crypto.py
+++ b/src/OpenSSL/crypto.py
@@ -7,7 +7,7 @@
 from six import (
     integer_types as _integer_types,
     text_type as _text_type,
-    PY3 as _PY3)
+    PY2 as _PY2)
 
 from cryptography import x509
 from cryptography.hazmat.primitives.asymmetric import dsa, rsa
@@ -377,7 +377,7 @@
     """
     _curves = None
 
-    if _PY3:
+    if not _PY2:
         # This only necessary on Python 3.  Morever, it is broken on Python 2.
         def __ne__(self, other):
             """