Deprecate Py_HASH_EXTERNAL as it cannot be used for `--with-hash-algorithm`
Bug report
Bug description:
In PEP-456, we introduced the notion of interchangeable hash functions for strings, bytes and memoryviews. Historically, Python used FNV as its hash algorithm, switched to SipHash2-4 in 3.4 before making SipHash1-3 the default in 3.11. It is however possible for embedders to implement their own hashing algorithms but this requires Python to be configured with --with-hash-algorithm=external and linking Python with the appropriate additional library.
Unfortunately, it's not possible to choose external and the only workaround would be to pass -DPy_HASH_ALGORITHM 0 but this is neither documented nor tested (it may not even work if the macro is still defined afterwards).
CPython versions tested on:
CPython main branch
Operating systems tested on:
No response