Utilize pre-commit to help devs follow style guidelines

[tim-schilling]

Closes #1485

@matthiask one thing that requires a close look is .pre-commit-config.yaml. Let me know if I should remove any that you think are overly strict. I picked from https://pre-commit.com/hooks.html and matched what we had. I did introduce doc8 since it seems like a positive change.

[codecov]

Codecov Report

Merging #1495 (30e5149) into main (3ed59cb) will increase coverage by 0.02%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##             main    #1495      +/-   ##
==========================================
+ Coverage   86.58%   86.61%   +0.02%     
==========================================
  Files          35       35              
  Lines        1864     1868       +4     
  Branches      262      262              
==========================================
+ Hits         1614     1618       +4     
  Misses        178      178              
  Partials       72       72              

Impacted Files	Coverage Δ
debug_toolbar/panels/cache.py	82.26% <0.00%> (ø)
debug_toolbar/management/commands/debugsqlshell.py	100.00% <100.00%> (ø)
debug_toolbar/panels/history/__init__.py	100.00% <100.00%> (ø)
debug_toolbar/panels/sql/__init__.py	100.00% <100.00%> (ø)
debug_toolbar/panels/templates/__init__.py	100.00% <100.00%> (ø)

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[matthiask]

I like it, especially the strictness of it :)

Regarding F401: You could add e.g. __all__ = ["SQLPanel"] to avoid the need to silence this particular error, but it's definitely fine as it is.

[tim-schilling]

I'm glad I forced the specific flake8 warnings, TIL. I'll make that change and merge it in.