Swap to pyproject.toml setup

[CodyCBakerPhD]

Modern Pythonic practice has moved away from setup.py +/- .cfg towards pyproject.toml

This updates the package accordingly

[CodyCBakerPhD]

I copied over these two RTD files to start a smooth transition to keeping the files there instead of the outer level to reduce clutter

It works fine as long as the settings on the RTD admin page are adjusted, which is why I will wait until after this PR is merged to do that and then remove the files still at the outer level (so it doesn't break on the other current PRs)

[CodyCBakerPhD]

This also removes Python 3.8 support, which ends its security upgrades next month

[CodyCBakerPhD]

This will take a bit more work - will splinter a few items into smaller PRs

[CodyCBakerPhD]

@stephprince OK this is ready now

I'll remove the RTD files on the outer level in a followup so the docs have a smooth transition

[stephprince]

Looks good to me. Added a couple small suggestions.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 84.96%. Comparing base (e3cc7e2) to head (d7d5597).

Files with missing lines	Patch %	Lines
src/nwbinspector/version/__init__.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #507      +/-   ##
==========================================
+ Coverage   79.56%   84.96%   +5.39%     
==========================================
  Files          48       47       -1     
  Lines        1390     1390              
==========================================
+ Hits         1106     1181      +75     
+ Misses        284      209      -75     

Files with missing lines	Coverage Δ
src/nwbinspector/_nwbinspector_cli.py	89.18% <100.00%> (+5.40%)	⬆️
src/nwbinspector/version/__init__.py	0.00% <0.00%> (ø)

... and 4 files with indirect coverage changes