Enforce LOW_S after Nov, 13
Summary: This is something that was in the plans, so let's just do it.
make check ../qa/pull-tester/rpc-tests.py
NB: prioritise_transaction.py fails but it doesn't looks related to this diff.
Reviewers: schancel, freetrader, #bitcoin_abc
Reviewed By: schancel, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D616