chore: add pytest-cov and CI workflow

Add GitHub Actions workflow for running tests with coverage on Python 3.8 and 3.11.

Configure pytest-cov, coverage thresholds and expand testpaths in pyproject.toml.

Update .gitignore for .ruff_cache, .mypy_cache, *.egg-info and .coverage.

.gitignore

@@ -1,6 +1,8 @@
 .idea
 .vscode
 .pytest_cache
+.ruff_cache
+.mypy_cache
 .jupyter
 *.ipynb
 *.ipynb_checkpoints
@@ -10,5 +12,7 @@ __pycache__
 .venv
 *.code-workspace
 *.iml
+*.egg-info
+.coverage
 kiauh.cfg
 klipper_repos.txt