Suppress third party warnings in pytest

[stefsmeets]

Is your feature request related to a problem? Please describe. Currently running pytest generates a large number of warnings from third-party modules, i.e.

tests/functional/test_multimodel.py: 300 warnings
  /home/stef/miniconda3/envs/esmvaltool/lib/python3.8/site-packages/iris/fileformats/cf.py:221: DeprecationWarning: tostring() is deprecated. Use tobytes() instead.
    return tuple((attr, self.getncattr(attr)) for attr in

These are unhelpful, and frankly not our responsibility, but they distract from testing and obscure the warnings that we can fix. I would like to add some filters to remove those warnings as described here: https://stackoverflow.com/questions/58399870/pytest-suppress-deprecationwarning-from-specific-3rd-party-modules

Would you be able to help out? 👍👍

[schlunma]

Moving this to v2.6 since there is not open PR yet.