contrib: correct version check

Summary:

[ not(major >= 3 and minor >= 6) ] fails for '4.0'

This is a backport of core#13699

D1120 introduced f-strings which require python >= 3.6

Test Plan: Run contrib/zmq/zmq_pub.py with python >= 3.6 and verify that it doesn't print the error message about the python version.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10308