Add tox and change coverage configuration

[egpbos]

This PR changes two things:

1. It adds tox support, which provides a fix for #38 and is just generally nice to have (I personally always add it).
2. It takes out the coverage flags from the default pytest settings. The coverage flags were breaking IDE test discovery. Instead, coverage can just be run separately with coverage or by supplying the flags manually, like is being done in the sonarcloud Actions workflow.

Edit: also updated a broken link.

[egpbos]

@sverhoeven Added documentation in 6d78d07.