[electrum] fix flake8-comprehensions warnings and enable flake8 for electrum

Summary:
Fixes the warnings listed here: https://github.com/adamchainz/flake8-comprehensions#rules

Also remove two unused imports in dnssec.py to fix flake8 error introduced in D14094 when I deleted the code using these imports.

Depends on D14106

Test Plan:

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

Run electrum-abc, enable cash fusion.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14107