Migrate gitian-build.sh to python

Summary:
Backport of core PR13623.
https://github.com/bitcoin/bitcoin/pull/13623/files

The python script got added in D2293 with a more recent version of the
script, so it is untouched. But the shell script is getting removed as
it is a duplicate.

Test Plan:

NOP();

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3951