Use pure python for functional tests schnorr computation

Summary:
This is a follow-up for D5965.
It converts our current schnorr.py library to use the framework from
key.py and remove openssl as a dependency for the functional tests.

Test Plan:

ninja check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7487