bpo-40791: Use CRYPTO_memcmp() for compare_digest (#20456)

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.

Modules/_operator.c

@@ -785,6 +785,8 @@ _operator_length_hint_impl(PyObject *module, PyObject *obj,
     return PyObject_LengthHint(obj, default_value);
 }
 
+/* NOTE: Keep in sync with _hashopenssl.c implementation. */
+
 /*[clinic input]
 _operator._compare_digest = _operator.eq