Export keying material support (#725)

* added method to export keying material from an ssl connection

* updated tests to use bytestrings to avoid breaking python3 tests

* added additional comments to test

* simplify export_keying_material

* add changelog

* address review feedback
diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 3f5590f..ae094cf 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -25,6 +25,8 @@
 
 - Fixed a potential use-after-free in the verify callback and resolved a memory leak when loading PKCS12 files with ``cacerts``.
   `#723 <https://github.com/pyca/pyopenssl/pull/723>`_
+- Added ``Connection.export_keying_material`` for RFC 5705 compatible export of keying material.
+  `#725 <https://github.com/pyca/pyopenssl/pull/725>`_
 
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