Update the contribution docs

Author: SmileyChris

CONTRIBUTING.rst

@@ -98,6 +98,6 @@ Before you submit a pull request, check that it meets these guidelines:
 2. If the pull request adds functionality, the docs should be updated. Put
    your new functionality into a function with a docstring, and add the
    feature to the list in README.rst.
-3. The pull request should work for Python 2.7, 3.5, 3.6, 3.7 and 3.8. Check
+3. The pull request should work for Python 2.7, 3.6, 3.7, 3.8 and 3.9. Check
    https://travis-ci.org/graphql-python/graphql_ws/pull_requests
    and make sure that the tests pass for all supported Python versions.