bpo-45434: Remove pystrhex.h header file (GH-28923)

Move Include/pystrhex.h to Include/internal/pycore_strhex.h.
The header file only contains private functions.

The following C extensions are now built with Py_BUILD_CORE_MODULE
macro defined to get access to the internal C API:

* _blake2
* _hashopenssl
* _md5
* _sha1
* _sha3
* _ssl
* binascii
22 files changed