contrib: correct version check
contrib: correct version check

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

This is a backport of [[https://github.com/bitcoin/bitcoin/pull/13699 | 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

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