coverage warning

[ThomasWaldmann]

iirc since introducing pytest-xdist, there is this warning:

==> wheezy64: Coverage.py warning: Module borg was previously imported, but not measured.

[ThomasWaldmann]

Found why it might happen and how to get rid of it there:

https://github.com/nedbat/coveragepy/blob/coverage-5.0a5/tests/test_process.py#L913