Update Python 2 doc link to Python 3
Python 3 docs are better maintained and are the future of the Python
project.
@@ -227,7 +227,7 @@ Alternatively you can capture the warnings with the standard :mod:`logging` modu
Finally, you can suppress the warnings at the interpreter level by setting the
``PYTHONWARNINGS`` environment variable or by using the
-`-W flag <https://docs.python.org/2/using/cmdline.html#cmdoption-W>`_.
+`-W flag <https://docs.python.org/3/using/cmdline.html#cmdoption-w>`_.
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