bpo-44145: Release the GIL around HMAC_Update. (GH-26157)

It was always meant to be released for parallelization.
This now matches the other similar code in the module.

Thanks michaelforney for noticing!
(cherry picked from commit c10392e7ddb3eafbd11e9ffe335c07648426715f)

Co-authored-by: Gregory P. Smith <greg@krypto.org>