gh-141226: Deprecate PEP-456 support for embedders (#141287)

Deprecate PEP-456 [1] support for providing an external definition
of the string hashing scheme. Removal is scheduled for Python 3.19.

Previously, embedders could define the ``Py_HASH_ALGORITHM`` macro to be
``Py_HASH_EXTERNAL`` [2] to indicate that the hashing scheme was provided
externally but this feature was undocumented, untested and most likely
unused.

[1]: https://peps.python.org/pep-0456/
[2]: https://peps.python.org/pep-0456/#hash-function-selection