Add the python extension to the macdeployqtplus script

Summary:
This is a python script, so let's call it like a python script.
This causes the script to be parsed by all the python linters, so this
diff updates it accordingly. This is a lot of changes but it's
formatting only.

Depends on D7967 and D7968.

Test Plan:

arc lint -- contrib/macdeploy/macdeployqtplus.py
ninja osx-dmg

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7969