on new FIPS compliant Python HMAC is implemented as a thin wrapper around
openssl implementation of HMAC, that makes it impossible to add a
field to the hmac object, but they actually do have the block_size
field and set it to correct values

as new python uses OpenSSL to implement HMAC (so that it is FIPS
compliant), using MD5 for HMAC will not work

as we need it for older ciphers (and to verify that those old ciphers
don't work in FIPS mode) we need to overwrite that limitation