feature #1566 Python 3.8.6 documentation

[georgemccabe]

Pull Request Testing

• Describe testing already performed for these changes:
  

• Recommend testing for the reviewer(s) to perform, including the location of input datasets, and any additional instructions:
  

Review documentation changes

https://metplus.readthedocs.io/en/feature_1566_py3.8.6_docs/Users_Guide/overview.html
https://metplus.readthedocs.io/en/feature_1566_py3.8.6_docs/Users_Guide/installation.html#software-requirements

• Do these changes include sufficient documentation updates, ensuring that no errors or warnings exist in the build of the documentation? [Yes]

• Do these changes include sufficient testing updates? [Yes]

• Will this PR result in changes to the test suite? [No]
  
  If yes, describe the new output and/or changes to the existing output:
  

• Please complete this pull request review by 8/2/2022.
  

Pull Request Checklist

See the METplus Workflow for details.

• Add any new Python packages to the METplus Components Python Requirements table.
• Review the source issue metadata (required labels, projects, and milestone).
• Complete the PR definition above.
• Ensure the PR title matches the feature or bugfix branch name.
• Define the PR metadata, as permissions allow.
  Select: Reviewer(s)
  Select: Organization level software support Project or Repository level development cycle Project
  Select: Milestone as the version that will include these changes
• After submitting the PR, select Linked issues with the original issue number.
• After the PR is approved, merge your changes. If permissions do not allow this, request that the reviewer do the merge.
• Close the linked issue and delete your feature or bugfix branch from GitHub.

[lisagoodrich]

All changes look good to me. All instances of Python 3.6 to have been changed to Python3.8