Add pytest to dev dependencies;

upgrade dev dependencies;
remove unnecessary main test script entrypoint;
remove unnecessary line from `.gitignore`;
put repo description variable into top-level `__init__.py`;
introduce syntax-error as a reminder to fix copyright;
add CSS fix for broken inline-code segments;
update `index.md` layout;
reorder linting script calls and add a bit more output;
support Python 3.8+ by default;
fix wrong dependency file reference in `pyproject.toml`;
add `pytest` config to `pyproject.toml`;
call `pytest` in `coverage` config;
add a few sensible rule-ignores to `ruff` config

tests/__main__.py

@@ -1,13 +0,0 @@
-import sys
-import unittest
-
-
-if __name__ == "__main__":
-    try:
-        pattern = sys.argv[1]
-    except IndexError:
-        pattern = "test*.py"
-    test_suite = unittest.defaultTestLoader.discover(".", pattern=pattern)
-    test_runner = unittest.TextTestRunner(resultclass=unittest.TextTestResult)
-    result = test_runner.run(test_suite)
-    sys.exit(not result.wasSuccessful())