Tune C source generation

[asvetlov]

Don't store _find_headers.c in the source tree but always regenerate it if needed.

[codecov-io]

Codecov Report

Merging #5164 into master will decrease coverage by 0.10%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5164      +/-   ##
==========================================
- Coverage   97.56%   97.45%   -0.11%     
==========================================
  Files          43       43              
  Lines        8780     8779       -1     
  Branches     1412     1412              
==========================================
- Hits         8566     8556      -10     
- Misses        100      112      +12     
+ Partials      114      111       -3     

Flag	Coverage Δ
#unit	97.45% <ø> (-0.11%)	⬇️

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Impacted Files	Coverage Δ
aiohttp/hdrs.py	100.00% <ø> (ø)
aiohttp/frozenlist.py	95.83% <0.00%> (-2.09%)	⬇️
aiohttp/http_writer.py	97.36% <0.00%> (-1.76%)	⬇️
aiohttp/http_parser.py	95.81% <0.00%> (-0.89%)	⬇️
aiohttp/http_websocket.py	98.37% <0.00%> (-0.28%)	⬇️
aiohttp/helpers.py	96.45% <0.00%> (-0.24%)	⬇️

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[github-actions]

💔 Backport was not successful

The PR was attempted backported to the following branches:

• ❌ 3.7: Commit could not be cherrypicked due to conflicts
• ❌ 3.8: Commit could not be cherrypicked due to conflicts