Add missing references

Author: Mariatta

pep-0581.rst

@@ -280,6 +280,11 @@ References
 .. [#gh-1501] CPython GitHub Pull request 1505
    (https://github.com/python/cpython/pull/1505)
 
+.. [#github-cpython1] Python-committers email
+   (https://mail.python.org/pipermail/python-committers/2015-December/003642.html)
+
+.. [#github-cpython-2] Python-committers email
+   (https://mail.python.org/pipermail/python-committers/2015-December/003645.html)
 
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