Relocate everything to make packaging work

[webbnh]

This PR is an attempt to reconcile the existing problems with packaging, installation, and PYTHONPATH.

This PR takes the existing relay subdirectory, which contains the __init__.py file and therefore defines the relay Python package, and moves it into a src subdirectory. (It also moves the existing tests directory which was a subdirectory of relay and makes it a peer of the new src directory at the top level; and, it moves the existing relay/requirements.txt file up a level to the src subdirectory.) With the commensurate changes to the setup.cfg file, the packaging now includes the file-relay sources as well as the relay stub-executable, so that, when the package is installed, the relay package is copied into the site-packages directory as it should be and it is available without explicitly defining PYTHONPATH. I'm not exactly sure what magic I've wrought here, but everything now seems to work as expected.

This PR also updates the ci.yml workflow to reflect the new directory structure (e.g., the locations of the respective requirements.txt files have changed, and the tests and sources are no longer in the same directory tree, so we have to request coverage for them separately) and to Pip-install the relay package from its sources (and to do it all in a single command). This allows the unit tests to work without a PYTHONPATH definition, testing the package as any other client would.

[dbutenhof]

Once you fix the "flake8y" code, I'll be interested in seeing whether the tests run. 😁

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01% ⚠️

Comparison is base (74ae60d) 98.52% compared to head (91a05ab) 98.52%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #8      +/-   ##
==========================================
- Coverage   98.52%   98.52%   -0.01%     
==========================================
  Files           2        2              
  Lines         543      541       -2     
==========================================
- Hits          535      533       -2     
  Misses          8        8              

Files Changed	Coverage Δ
src/relay/relay.py	100.00% <ø> (ø)
tests/test_relay.py	97.95% <100.00%> (ø)

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[webbnh]

Patch coverage: 100.00% and project coverage change: -0.01% ⚠️

Such is the power of statistics: by removing three previously covered lines, I increased the fraction of non-covered lines, which therefore decreased the overall "coverage". 😆

In any case, this change is now ready for review.