chore: drop flake8 to only use ruff

[mkniewallner]

PR Checklist

• A description of the changes is added to the description of this PR.
• If there is a related issue, make sure it is linked to this PR.
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

Description of changes

ruff now supports most flake8-simplify rules (astral-sh/ruff#998). Since the last ones will most likely be implemented sooner than later, and all other rules we use for flake8 are implemented, it might be time to fully rely on ruff and drop flake8.

Also taking the occasion to add flake8-builtins rules.

[codecov-commenter]

Codecov Report

Merging #268 (6477e52) into main (0993fc0) will decrease coverage by 0.0%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main    #268     +/-   ##
=======================================
- Coverage   95.7%   95.7%   -0.1%     
=======================================
  Files         32      32             
  Lines        922     920      -2     
  Branches     193     192      -1     
=======================================
- Hits         883     881      -2     
  Misses        26      26             
  Partials      13      13             

Impacted Files	Coverage Δ
deptry/imports/extract.py	100.0% <100.0%> (ø)

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