[issue40302] Add pycore_byteswap.h internal header file with _Py_bswap32() function

[Sergey Fedoseev]

Change by Sergey Fedoseev <fedoseev.sergey at gmail.com>:


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nosy: +sir-sigurd
nosy_count: 3.0 -> 4.0
pull_requests: +19338
pull_request: https://github.com/python/cpython/pull/15659

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<https://bugs.python.org/issue40302>
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