[WIP] bpo-35134: Move header files to Include/cpython/

[vstinner]

The whole cellobject.h API is excluded from the limited C API, so
move the header file to Include/cpython/ directory. Since
cellobject.h is included by the main Python.h: users should not be
affected by this change.

https://bugs.python.org/issue35134

[codecov]

Codecov Report

Merging #18490 into master will increase coverage by 0.00%.
The diff coverage is n/a.

@@ Coverage Diff @@## master #18490 +/- ## ========================================= Coverage 82.11% 82.12% ========================================= Files 1956 1955 -1 Lines 589183 583854 -5329 Branches 44448 44449 +1 ========================================= - Hits 483833 479488 -4345 + Misses 95701 94722 -979 + Partials 9649 9644 -5 

Impacted Files	Coverage Δ
Lib/distutils/tests/test_bdist_rpm.py	30.00% <0.00%> (-65.00%)	⬇️
Lib/distutils/command/bdist_rpm.py	7.63% <0.00%> (-56.88%)	⬇️
Lib/test/test_urllib2net.py	76.92% <0.00%> (-13.85%)	⬇️
Lib/test/test_smtpnet.py	78.57% <0.00%> (-7.15%)	⬇️
Lib/ftplib.py	63.85% <0.00%> (-6.06%)	⬇️
Lib/test/test_ftplib.py	87.11% <0.00%> (-4.72%)	⬇️
Tools/scripts/db2pickle.py	17.82% <0.00%> (-3.97%)	⬇️
Tools/scripts/pickle2db.py	16.98% <0.00%> (-3.78%)	⬇️
Lib/test/test_socket.py	71.94% <0.00%> (-3.77%)	⬇️
Lib/test/test_asyncio/test_base_events.py	91.84% <0.00%> (-3.30%)	⬇️
... and 330 more

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[vstinner]

This change break C extension modules which explicitly include on these moved header files.

A safer approach would be to keep .h files in Include/ but move their content into a new Include/cpython/ file.

I close this PR.