Add KeySeq class

[khaeru]

Also add Key.remove_tag, Key.__sub__.

Housekeeping:

• Inline pre-commit action steps, since the original is no longer maintained (cf. pre-commit/action@v3.0.0 Depends on actions/cache@v3 which is using Node 16 which is now deprecated pre-commit/action#189, Update action checkout, cache, python pre-commit/action#188)

PR checklist

• Checks all ✅
• Update documentation
• Update doc/whatsnew.rst

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c095443) 100.00% compared to head (4a22b1c) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #126   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           50        50           
  Lines         4262      4358   +96     
=========================================
+ Hits          4262      4358   +96     

Files	Coverage Δ
genno/__init__.py	100.00% <100.00%> (ø)
genno/core/key.py	100.00% <100.00%> (ø)
genno/testing/__init__.py	100.00% <ø> (ø)
genno/tests/core/test_key.py	100.00% <100.00%> (ø)