[electrum] add missing encodings to open() calls and enable linter

Summary: As per title

Test Plan:

arc lint --everything
cd electrum
python -m electrumabc.tests

Build for windows and run from command line with arg -v (to test the winconsole encoding)
Try also wine dist/ElectrumABC-5.2.4.exe -v on linux.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14112