Allow CPython to build against cryptography libraries lacking BLAKE2 support

[WillChilds-Klein]

Feature or enhancement

Proposal:

As part of a series of changes discussed on the python Ideas board, this issue proposes placing guards around references to BLAKE2's NIDs. This would allow cryptography libraries lacking full BLAKE2 support to be used with CPython.

Has this already been discussed elsewhere?

I have already discussed this feature proposal on Discourse

Links to previous discussion of this feature:

https://discuss.python.org/t/support-building-ssl-and-hashlib-modules-against-aws-lc/44505

Linked PRs

• gh-117233: Detect libcrypto BLAKE2, Shake, SHA3, and Truncated-SHA512 support at hashlib build time #117234
• [3.12] gh-117233: Detect support for several hashes at hashlib build time (GH-117234) #117767