bpo-35081: Move bytes_methods.h to the internal C API

[vstinner]

Move the bytes_methods.h header file to the internal C API as
pycore_bytes_methods.h: it only contains private symbols (prefixed by
"_Py"), except of the PyDoc_STRVAR_shared() macro.

https://bugs.python.org/issue35081

[codecov]

Codecov Report

Merging #18492 into master will increase coverage by 1.08%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master   #18492       +/-   ##
===========================================
+ Coverage   82.11%   83.19%    +1.08%     
===========================================
  Files        1956     1571      -385     
  Lines      589183   414643   -174540     
  Branches    44448    44449        +1     
===========================================
- Hits       483833   344981   -138852     
+ Misses      95701    60016    -35685     
+ Partials     9649     9646        -3     

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 430 more

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