Add python3 to list of dependencies on some platforms

Summary:

python3 is required for running the unit tests on macOS, Ubuntu and
Fedora.

Without python3 installed, 'make check' fails because
/test/util/bitcoin-util-test.py fails to find python3.

Backport of core PR11136
https://github.com/bitcoin/bitcoin/pull/11136/files

Depends on D3904.

Test Plan: Read it.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3905