CI workflow setup: lint + test

[ravwojdyla]

This PR depends on #151, which should be reviewed and merged first. This PR adda a CI workflow which runs both lint and tests.

Summary:

• update relevant actions to avoid GH warnings
• run both lint and test at the same time
• tests run on python 3.9, 3.10 and 3.11
• adds badge to the readme

[ravwojdyla]

There appears to be some failures in tests/test_plink.py, they do not look related to this PR:

FAILED tests/test_plink.py::TestEqualSgkit::test_simulated_example - AttributeError: 'int' object has no attribute 'item'
FAILED tests/test_plink.py::TestExample::test_sgkit - AttributeError: 'int' object has no attribute 'item'

Those tests fail consistently across python versions ^, I suppose they could be fixed in a separate PR?

[jeromekelleher]

LGTM

Yep, let's go ahead and merge this while failing, and can fix with a follow-up

[ravwojdyla]

@jeromekelleher this should be good to go 🙏