tests: add pytest-cov .coveragerc file

When using pytest-cov this file can be used to configure the coverage report.
See https://coverage.readthedocs.io/en/latest/config.html for details.
So when you start py.test like this

py.test --cov-config .coveragerc --cov=kallithea --cov-report xml

the settings in this file will be used.