Add Python 3.9 support to CI

[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.
• [x] 🛠️ All tests pass when run locally (run tox or make test to find out!).
• [x] 🧹 All linting checks pass when run locally (run tox -e lint or make lint to find out!).
• [ ] 📚 Updated documentation for changed code.
• [x] ⏩ 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

[codecov-commenter]

Codecov Report

Merging #824 into master will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #824   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10         9    -1     
  Lines         1776      1770    -6     
  Branches       304       304           
=========================================
- Hits          1776      1770    -6     

Impacted Files	Coverage Δ
arrow/__init__.py

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