Add more specific coverage configuration

pyproject.toml:
Set specific source for `toolscoverage.paths` (as we only provide one
module).
Set `tools.coverage.run.relative_files` to true (although it is still
bug riddled and does not seem to work as advertized in regards to xml
output: https://github.com/nedbat/coveragepy/issues/963,
https://github.com/nedbat/coveragepy/issues/1147).
Set `tool.coverage.xml.output`, so we don't have to provide it on the
commandline.

Makefile:
Do not provide an output file to the coverage xml call, as we do that in
configuration now.