Reduce API surface for BLAKE2 to match OpenSSL semantics

I want to reduce BLAKE2 in hashlib to a minimal API matching OpenSSL. More precisely, BLAKE2 is kept without SIMD and matches the API of OpenSSL’s BLAKE2 (digest, MAC): data and digest_size for the hash, and salt/person with a key. Tree hashing, salt/person without a key, and HMAC-BLAKE2 will be deprecated in 3.16 and removed in 3.21. If necessary, the deprecation period will be extended to a few more releases but I really want to get rid of the annoying API surface. BLAKE2 is already a keyed h...