Declare support for Python 3.10

[hugovk]

Pull Request Checklist

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

• [n/a] 🧪 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

Add Trove classifier for Python 3.10. Follow on from #1049.

[codecov]

Codecov Report

Merging #1053 (9652cd9) into master (1e69992) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master     #1053   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines         2218      2218           
  Branches       348       348           
=========================================
  Hits          2218      2218           

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[jadchaar]

LGTM