ci: Quote GitHub Action python-version number as YAML strings

[matthewfeickert]

Description

As preventative actions, quote the python-version numbers in CI as without quotes YAML will treat 3.10 as a float and interpret it as 3.1 which is certainly not desired. :) This won't affect us until all backends are on Python 3.10, but it will keep us from having the CI fail weirdly in the future.

Also simplify the macos-latest inclusion with include keyword.

Please describe the purpose of this pull request in some detail. Reference and link to any relevant issues or pull requests.

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Quote all python-version numbers in CI. This is done as a preventive
action to set a YAML string precedence for when Python 3.10 is added
to testing, as without quotes YAML will treat `3.10` as a float and 
interpret it as `3.1`.
* Simplify the inclusion of the latest Python version on macos-latest
by using the `include` keyword in the CI strategy matrix to add it
instead of excluding all Python versions to not test.

[codecov]

Codecov Report

Merging #1707 (8735b3f) into master (9f9a6b4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1707   +/-   ##
=======================================
  Coverage   98.11%   98.11%           
=======================================
  Files          64       64           
  Lines        4249     4249           
  Branches      590      590           
=======================================
  Hits         4169     4169           
  Misses         46       46           
  Partials       34       34           

Flag	Coverage Δ
contrib	26.33% <ø> (ø)
doctest	61.04% <ø> (ø)
unittests	96.16% <ø> (ø)

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[matthewfeickert]

I'm going to merge this without asking for additional review only because it is quoting the version number and the CI still passes. I assume that people don't mind me not annoying them here with a trivial 👍, but let me know if people would prefer that in the future.