[electrum] add a basic test for setup.py

Summary:
This would have caught the bug fixed by D14258.

electrum-tests is run by CI on any change in the electrum/ directory, even if it is not touching python code.

Test Plan:
python test_runner.py

Try reintroducing an error in setup.py [README.md -> README.rst) and check that it fails and print the error tracebak.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14260