build: Require python 3.5

Summary:
Partial backport of core PR14954 (commit fab5a1e):
https://github.com/bitcoin/bitcoin/pull/14954/commits/fab5a1e0f492f920270c661954802749dfac5fa1

Note: the depends README change is not backported because it has
dependencies that will be added later.

Test Plan:

./test/functional/test_runner.py --extended

Run the gitian builds.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4372