pythongh-135234: improve _hashlib exceptions when reporting an OpenSSL function failure (python#135250)

- Refactor `get_openssl_evp_md_by_utf8name` error branches.
- Refactor `HASH.{digest,hexdigest}` computations.
- Refactor `_hashlib_HASH_copy_locked` and `locked_HMAC_CTX_copy`.

Author: picnixz

Misc/NEWS.d/next/Library/2025-06-08-11-11-07.gh-issue-135234.wJCdh0.rst

@@ -0,0 +1,3 @@
+:mod:`hashlib`: improve exception messages when an OpenSSL function failed.
+When memory allocation fails on OpenSSL's side, a :exc:`MemoryError` is
+raised instead of a :exc:`ValueError`. Patch by Bénédikt Tran.