Emit a warning on Python 2.6

CHANGELOG.rst

@@ -6,6 +6,10 @@ Changelog
 
 .. note:: This version is not yet released and is under active development.
 
+* Deprecated support for Python 2.6. At the time there is no time table for
+  actually dropping support, however we strongly encourage all users to upgrade
+  their Python, as Python 2.6 no longer receives support from the Python core
+  team.
 * Support :attr:`~cryptography.hazmat.primitives.serialization.Encoding.DER`
   serialization of public keys using the ``public_bytes`` method of
   :class:`~cryptography.hazmat.primitives.asymmetric.rsa.RSAPublicKeyWithSerialization`,

src/cryptography/__init__.py

@@ -4,6 +4,9 @@
 
 from __future__ import absolute_import, division, print_function
 
+import sys
+import warnings
+
 from cryptography.__about__ import (
     __author__, __copyright__, __email__, __license__, __summary__, __title__,
     __uri__, __version__
@@ -14,3 +17,10 @@
     "__title__", "__summary__", "__uri__", "__version__", "__author__",
     "__email__", "__license__", "__copyright__",
 ]
+
+if sys.version_info[:2] == (2, 6):
+    warnings.warn(
+        "Python 2.6 is no longer supported by the Python core team, please "
+        "upgrade your Python.",
+        DeprecationWarning
+    )