Update style testing

[WilliamJamieson]

Adds:

• fylnt testing (modernizes all strings)
• pyproject-fmt and validate-pyproject testing (lints the pyproject.toml file)

Removes:

• pyupgrade (ruff now fully replaces it)
• bandit (ruff now fully replaces it)

[codecov]

Codecov Report

Merging #165 (83b05a0) into main (1b40863) will increase coverage by 0.31%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #165      +/-   ##
==========================================
+ Coverage   98.53%   98.84%   +0.31%     
==========================================
  Files          52       52              
  Lines        1909     1909              
==========================================
+ Hits         1881     1887       +6     
+ Misses         28       22       -6     

Impacted Files	Coverage Δ
asdf_astropy/converters/transform/properties.py	91.11% <0.00%> (-8.89%)	⬇️
...tropy/converters/transform/tests/test_transform.py	96.41% <0.00%> (+0.71%)	⬆️
asdf_astropy/converters/transform/core.py	98.31% <0.00%> (+6.72%)	⬆️

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