gh-85283: Build md5 extension with limited C API

[vstinner]

• Replace _Py_strhex() with few lines of code.
• Replace _PyType_GetModuleState() with PyType_GetModuleState().

• Issue: [C API] Convert a few stdlib extensions to the limited C API (PEP 384) #85283

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📚 Documentation preview 📚: https://cpython-previews--110967.org.readthedocs.build/

[vstinner]

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.

[vstinner]

The _md5 extension is not used in practice:

vstinner@mona$ ./python
Python 3.13.0a1+ (heads/limited_md5-dirty:417055ae80, Oct 17 2023, 12:28:44) [GCC 13.2.1 20231011 (Red Hat 13.2.1-4)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import _md5
>>> h1=_md5.md5(b'Python')
>>> type(h1).__module__
'_md5'
>>> import hashlib
>>> 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.