[electrum] fix the minimum python version

Summary:
Make the minimum python version consistent between the setup.py and the CONTRIBUTING.md file (D15373)

Remove the long list of hard to maintain classifiers for supported python versions. This info is targeted at users of pypi.org, and we don't publish Electrum ABC releases there.

Use the python_requires argument to setup rather than ad-hoc code checking the version.
Depends on D17298

Test Plan:

python setup.py build --dry-run
pip install . --dry-run --break-system-packages

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17299