[electrum] don't hack sys.path in setup.py

Summary: This is inspired by https://github.com/Electron-Cash/Electron-Cash/pull/2662/commits/f98c8b274f532196c16a9fc3b7c1e2e1ea21e26b

Test Plan:

python setup.py --version --name --fullname --url --author
python setup.py build
rm -Rf build/

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14439