Refactor/tests

[ultrabug]

FYI @kamilkrzyskow

[ultrabug]

I've cleaned up and made every test file independent

There's still work to do, but at least we now have only ONE dummy mkdocs.yml with explicit mkdocs config on every test

[kamilkrzyskow]

Woah you work fast. Sorry for the holdup on my end and thanks for laying the groundwork for me. MkDocs 1.4.2 still supports Python 3.7, so maybe it's still worth supporting it as long as MkDocs does it too 🤔

[ultrabug]

MkDocs 1.4.2 still supports Python 3.7, so maybe it's still worth supporting it as long as MkDocs does it too thinking

I did not actually drop 3.7 support from pip's perspective, but I find it useless to keep running linters and QA stuff on it.