Do not install C sources in binary distributions

[musicinmybrain]

This does not affect source distributions, and Cython sources (.pyx) are
still installed.

What do these changes do?

Prevent C sources (_find_header.c, _find_header.h, _helpers.c, _http_parser.c, _http_writer.c, and _websocket.c) from being included in binary distributions such as wheels, without affecting Cython sources (_cparser.pxd, _find_header.pxd, _headers.pxi, _helpers.pyx, _http_parser.pyx, _http_writer.pyx, and _websocket.pyx) or source distributions.

Are there changes in behavior for the user?

Nothing changes except that the binary wheels are much smaller because they do not contain ~1.7MB of C sources.

Related issue number

Resolves #6399.

Checklist

• I think the code is well written
• Unit tests for the changes exist This doesn’t seem necessary.
• Documentation reflects the changes No changes seem necessary.
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
  • This contribution is trivial, so this doesn’t seem necessary, but I can add it if desired.
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[webknjaz]

Not sure how I feel about this in general but here's some actionable feedback for now.

[codecov]

Codecov Report

Merging #6400 (db818a0) into master (dcb44a9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #6400   +/-   ##
=======================================
  Coverage   93.34%   93.34%           
=======================================
  Files         104      104           
  Lines       30456    30456           
  Branches     3064     3064           
=======================================
  Hits        28429    28429           
  Misses       1850     1850           
  Partials      177      177           

Flag	Coverage Δ
unit	93.26% <ø> (ø)

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[webknjaz]

Not sure how I feel about this in general but here's some actionable feedback for now.

Based on Andrew's comments in the issue, this will be good to go once those comments are addressed.

[musicinmybrain]

https://github.com/aio-libs/aiohttp/runs/4458544871?check_suite_focus=true#step:10:2941

E ResourceWarning: unclosed transport <_SelectorSocketTransport closing fd=46>

Doesn’t look like the CI failure has anything to do with the PR.

[webknjaz]

Doesn’t look like the CI failure has anything to do with the PR.

That's probably a race condition in tests. It's not related to the CI itself but the tests could be improved, for sure. And of course, it's a separate effort/PR. I'm restarting the CI for now.

[patchback]

Backport to 3.8: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.8/e1ed247dd3aa9a2fc3cd6d87b4845849da6ee494/pr-6400

Backported as #6402

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

[patchback]

Backport to 3.9: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.9/e1ed247dd3aa9a2fc3cd6d87b4845849da6ee494/pr-6400

Backported as #6403

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.