Backend-specific subclasses of PhysicalKey

[sir-sigurd]

Description

This change is mostly for avoiding annoying if pk.is_local() and to ease addition of new backends when/if we decide to do it.

This is a POC, I'll finish it if this change is generally approved.

TODO

• [ ] Unit tests
• [ ] Documentation
  • [ ] Run build.py for new docstrings
• [ ] Changelog entry

[codecov-io]

Codecov Report

Merging #1982 (21173fa) into master (49ee54b) will increase coverage by 0.02%. The diff coverage is 75.65%.

@@            Coverage Diff             @@
##           master    #1982      +/-   ##
==========================================
+ Coverage   90.40%   90.42%   +0.02%     
==========================================
  Files          64       64              
  Lines        7792     7821      +29     
==========================================
+ Hits         7044     7072      +28     
- Misses        748      749       +1     

Flag	Coverage Δ
api-python	89.39% <75.65%> (+0.04%)	:arrow_up:
lambda	92.40% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
api/python/quilt3/backends/s3.py	67.59% <59.41%> (-30.41%)	:arrow_down:
api/python/quilt3/backends/base.py	89.72% <92.30%> (ø)
api/python/quilt3/backends/local.py	95.76% <94.11%> (-2.42%)	:arrow_down:
api/python/quilt3/data_transfer.py	88.86% <100.00%> (+7.46%)	:arrow_up:
api/python/quilt3/packages.py	92.48% <100.00%> (+0.02%)	:arrow_up:
api/python/quilt3/util.py	81.52% <100.00%> (-1.16%)	:arrow_down:
api/python/tests/integration/test_workflows.py	100.00% <100.00%> (ø)
api/python/tests/test_data_transfer.py	100.00% <100.00%> (ø)

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[drernie]

Doesn't seem to have been finished; plan to handle via Python path lib.