[electrum] rename scripts with .py extension, and fix misc lint errors

Summary:
With the .py extension, these files will now be under the watch of our linters.

Fix two remaing linter issues: a missing #!/usr/bin/env python3 shebang and a missing encoding when calling open(...)

Depends on D15359

Test Plan:
contrib/source-control-tools/patch-recipes/update-electrum-checkpoints.sh

arc lint -- electrum/scripts/*py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15364