Updating readme to reflect support for Python 3.9 (#545)

According to the badge on the README Python 3.9 is supported and 3.9 is tested in CI, but it is not listed as being supported in the text of the README so I added it there.

Co-authored-by: Sambhav Kothari <sambhavs.email@gmail.com>

Author: sanders41

README.rst

@@ -32,7 +32,7 @@ docstring conventions.
 `PEP 257 <http://www.python.org/dev/peps/pep-0257/>`_ out of the box, but it
 should not be considered a reference implementation.
 
-**pydocstyle** supports Python 3.6, 3.7 and 3.8.
+**pydocstyle** supports Python 3.6, 3.7, 3.8 and 3.9.
 
 
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