remove required version of python3

don't remember why I required that but it is now breaking since the GitHub workflows have been updated to later versions of Ubuntu and Python

.github/workflows/docs.yml

@@ -37,8 +37,6 @@ jobs:
     # sphinx is a python package, so we need to setup python on this runner
     - name: Setup Python for Sphinx
       uses: actions/setup-python@v2
-      with:
-        python-version: 3.6
 
     # Runs sphinx-apidoc and sphinx-build in the docs/ directory
     #   sphinx-apidoc requires the python files to be packaged together