[otbn] Silence mypy error in yaml_parse_helpers.py
[otbn] Silence mypy error in yaml_parse_helpers.py

There might be a cleverer way to tell mypy the base class here, but I
don't see much point. Just tell it to be quiet!

Signed-off-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan