Fix pypy CI issues and begin testing against Python 3.10

[jadchaar]

Pull Request Checklist

Thank you for taking the time to improve Arrow! Before submitting your pull request, please check all appropriate boxes:

• 🧪 Added tests for changed code.
• 🛠️ All tests pass when run locally (run tox or make test to find out!).
• 🧹 All linting checks pass when run locally (run tox -e lint or make lint to find out!).
• 📚 Updated documentation for changed code.
• ⏩ Code is up-to-date with the master branch.

If you have any questions about your code changes or any of the points above, please submit your questions along with the pull request and we will try our best to help!

Description of Changes

Pypy3 builds recently began failing on our macOS CI runners. Turns out that this is because macOS 11 runners beginning to be used and they lack support for pypy3.6. I needed to update our workflow YAML to manually invoke pypy3.7 to fix this issue.

While I was at it, I also added support for testing against python 3.10 since it is in the final RC stages and will be released very soon.

[codecov]

Codecov Report

Merging #1045 (7dce65a) into master (2f54cb8) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master     #1045   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines         2208      2208           
  Branches       348       348           
=========================================
  Hits          2208      2208           

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