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

.gitignore

@@ -18,8 +18,5 @@
 __pycache__/
 *.py[cod]
 
-# Testing:
-/.coverage
-
 # Miscellaneous cache:
 .cache/