chore: migrate to hatchling

[Saransh-cpp]

• add .flake8 for flake8 config
• remove setup-cfg-fmt pre-commit
• remove setup.*
• migrate to hatchling

[codecov-commenter]

Codecov Report

Merging #223 (ef9dea9) into main (02d4655) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #223      +/-   ##
==========================================
+ Coverage   82.56%   82.58%   +0.01%     
==========================================
  Files          96       96              
  Lines       10475    10483       +8     
==========================================
+ Hits         8649     8657       +8     
  Misses       1826     1826              

Impacted Files	Coverage Δ
src/vector/_compute/spatial/z.py	100.00% <0.00%> (ø)
src/vector/_compute/planar/unit.py	100.00% <0.00%> (ø)
src/vector/_compute/spatial/eta.py	100.00% <0.00%> (ø)
src/vector/_compute/lorentz/beta.py	100.00% <0.00%> (ø)
src/vector/_compute/lorentz/unit.py	100.00% <0.00%> (ø)
src/vector/_compute/spatial/unit.py	100.00% <0.00%> (ø)
src/vector/_compute/lorentz/gamma.py	100.00% <0.00%> (ø)
src/vector/_compute/spatial/costheta.py	100.00% <0.00%> (ø)
src/vector/_compute/spatial/cottheta.py	100.00% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 02d4655...ef9dea9. Read the comment docs.

[Saransh-cpp]

Thank you for the review, @henryiii! Weirdly, the disassemble check fails due to no test_extras dependencies being installed -

WARNING: vector 0.1.dev1+g5255483 does not provide the extra 'test_extras'

[Saransh-cpp]

Turns out, pyproject.toml has a problem with extras having _ in their name.

[Saransh-cpp]

Works with -! Thanks, @henryiii!

[Saransh-cpp]

Should I go ahead and merge this?