Support Python 3.8+

[kurtmckee]

Description

This PR introduces the following changes:

• Drop support and testing for Python 2.7 and 3.7, both of which are EOL
• Add a Python version requirement to setup.cfg, so new packages published on PyPI cannot be installed on unsupported Python versions

Note

This PR should resolve existing CI failures. If it doesn't, I'll work to resolve those so CI passes cleanly.

If #136 merges first, I'll update this PR to resolve merge conflicts.

Pre-approval checklist (for submitter)

Please complete these steps

• Passes tests
• New tests for additional features or changed functionality
• My name and contribution added to contributors list (or if I'd rather opt out, I've said so in the PR)

[kurtmckee]

Friendly ping to review this PR.