build: depends: Switch to python3
Summary:
This completes the python 3 migration.
Backport of core PR15601:
https://github.com/bitcoin/bitcoin/pull/15601/files
Depends on D4363, D4364 and D4376.
Test Plan: Run the OSX Gitian builds.
Reviewers: #bitcoin_abc, deadalnix, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Differential Revision: https://reviews.bitcoinabc.org/D4377