Revamp PDBFixer testing

[swails]

• Use GitHub Actions to run a suite of tests
• Switch from nose to pytest (nose has not been supported in years)
• Download a PDB file and track it instead of fetching it during CI (RCSB has blocked testing frameworks in the past).

[swails]

I hadn't seen #222

This actually adds some testing, or it should at least.

[jchodera]

Also fixes #224.

We should still likely convert to using the MolSSI cookiecutter at some point down the road!

[swails]

Tests now pass: https://github.com/swails/pdbfixer/runs/3468227261?check_suite_focus=true

I'll clean up the branch trigger names and this should be good to merge. Then I'll proceed to the cleanups I need.

[swails]

@peastman this is ready to merge, and results in passed tests on Python 3.7 - Python 3.9

[peastman]

That looks excellent. Thank you!