update version of setup-python action

[lukewrites]

GitHub Actions now has Mac ARM runners! This is how we found out:

1. The version of setup-python we were using doesn't work on Apple Si runners
2. some of our unit tests were borked on Apple Si

This PR updates the action and also fixes the unit tests.

Fixing here

[pep8speaks]

Thanks for updating the PR.

• In the file tdvt/test/tdvt_test.py:

Line 409:121: E501 line too long (129 > 120 characters)
Line 386:121: E501 line too long (134 > 120 characters)
Line 372:121: E501 line too long (134 > 120 characters)
Line 362:121: E501 line too long (134 > 120 characters)
Line 347:121: E501 line too long (134 > 120 characters)

[pvanderknyff]

LGTM if tests pass