Replace _Py_strhex() with few lines of code.

For now, I prefer to not add such function to the public C API, since users may want many variants:

• Uppercase letters
• Space separator
• Get a bytes object instead of a str object

See Include/internal/pycore_strhex.h: there are 4 functions with different arguments.

The_md5 extension is not used in practice:

vstinner@mona$ ./pythonPython3.13.0a1+ (heads/limited_md5-dirty:417055ae80,Oct172023,12:28:44) [GCC13.2.120231011 (RedHat13.2.1-4)]onlinuxType"help","copyright","credits"or"license"formoreinformation.>>>import_md5>>>h1=_md5.md5(b'Python')>>>type(h1).__module__'_md5'>>>importhashlib>>>h2=hashlib.new('md5',b'Python')>>>type(h2).__module__'_hashlib'>>>h1.digest()==h2.digest()True

hashlib uses the OpenSSL implementation of MD5. The_md5 extension is only used when OpenSSL is missing, which should not be the case on Linux, Windows, macOS, FreeBSD, etc.