Add Python 3.9 support

[jadchaar]

Pull Request Checklist

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

• [x] 🧪 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!).
• [x] 📚 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

Adds support for Python 3.9 and pypy3 in tox.ini, Makefile, and Travis CI.

Github actions does not support development versions yet, but it is an open issue here: https://github.com/actions/setup-python/issues/20.

[codecov-commenter]

Codecov Report

Merging #801 into master will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #801   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines         1713      1713           
  Branches       295       295           
=========================================
  Hits          1713      1713           

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[jadchaar]

@jadchaar looks solid man, kinda lame that GH actions doesn't support 3.9 yet but also funny that Travis is bailing us out here.

Speaking of which has Travis CI run on this PR, I don't see it?

I don't know why GH is not showing it. It might that GH prioritizes GH Actions now over Travis, which would be kind of funny if true.

Here is the run: https://travis-ci.org/github/crsmithdev/arrow/builds/695468398. I had to find it by manually going to https://travis-ci.org/github/crsmithdev/arrow/.

GH Actions is still developing and expanding, so hopefully we will see this dev/nightly support before 3.10 :).