setup.cfg: Deactivate warning capture during tests

Since 3.8.x (and PEP 565), pytest displays DeprecationWarning and
PendingDeprecationWarning. Problem is that it does not filter the
module the warnings come from and display warnings from third party
libraries. Ignore them for now, details:

https://docs.pytest.org/en/latest/warnings.html#deprecationwarning-and-pendingdeprecationwarning

BuildGrid/buildgrid#36