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

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