New features:
* refactor `hashlib_helper.requires_hashdigest` in prevision of a future
`hashlib_helper.requires_builtin_hashdigest` for built-in hashes only
* add `hashlib_helper.requires_openssl_hashdigest` to request OpenSSL
hashes, assuming that `_hashlib` exists.
Refactoring:
* split hmac.copy() test by implementation
* update how algorithms are discovered for RFC test cases
* simplify how OpenSSL hash digests are requested
* refactor hexdigest tests for RFC test vectors
* typo fix: `assert_hmac_hexdigest_by_new` -> `assert_hmac_hexdigest_by_name`
Improvements:
* strengthen contract on `hmac_new_by_name` and `hmac_digest_by_name`
* rename mixin classes to better match their responsibility
Since we plan to introduce a built-in implementation for HMAC based on HACL*,
it becomes important for the HMAC tests to be flexible enough to avoid code
duplication.
In addition to the new layout based on mixin classes, we extend test coverage by
also testing the `__repr__` of HMAC objects and the HMAC one-shot functions.
We also fix the import to `_sha256` which, since gh-101924, resulted in some tests being
skipped as the module is no more available (its content was moved to the `_sha2` module).
Bénédikt Tran