Document that Python 3.6+ is needed for releasing a version.

README.rst

@@ -143,7 +143,7 @@ PyPy.
 Release
 -------
 
-Release requires Pipenv_. To bump the version run
+Release requires Pipenv_ and Python 3.6 or later. To bump the version run
 :code:`pipenv run bumpversion major|minor|patch` to update the version and git
 commit and tag the changes, then run :code:`git push --follow-tags` to push the
 git commit and tag. The GitLab CI will then build and upload a release to PyPI.