[electrum] make the python interpreter explicit when running setup.py

Summary:
A setup.py file is not usually executed on its own (./setup.py), people normally call (python setup.py)

Depends on D15446

Test Plan: electrum/test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15449