[PR #6400/e1ed247d backport][3.9] Do not install C sources in binary distributions

[patchback]

This is a backport of PR #6400 as merged into master (e1ed247).

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."

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (3.9@36960e6). Click here to learn what that means.
The diff coverage is n/a.

❗ Current head e1ed247 differs from pull request most recent head afbe7a0. Consider uploading reports for the commit afbe7a0 to get more accurate results

@@          Coverage Diff           @@
##             3.9    #6403   +/-   ##
======================================
  Coverage       ?   93.34%           
======================================
  Files          ?      104           
  Lines          ?    30456           
  Branches       ?     3064           
======================================
  Hits           ?    28429           
  Misses         ?     1850           
  Partials       ?      177           

Flag	Coverage Δ
unit	93.26% <0.00%> (?)

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