[3.9] bpo-40791: Use CRYPTO_memcmp() for compare_digest (GH-20456) (GH-20461)

[3.9] bpo-40791: Use CRYPTO_memcmp() for compare_digest (GH-20456) (GH-20461)

hashlib.compare_digest uses OpenSSL's CRYPTO_memcmp() function
when OpenSSL is available.

Note: The _operator module is a builtin module. I don't want to add
libcrypto dependency to libpython. Therefore I duplicated the wrapper
function and added a copy to _hashopenssl.c..
(cherry picked from commit db5aed931f8a617f7b63e773f62db468fe9c5ca1)

Co-authored-by: Christian Heimes <christian@python.org>