Tests fail with matplotlib 3.3.0rc1, because the `warn` argument was removed from `matplotlib.use`.


Version-Release number of selected component (if applicable):
python-MDAnalysis-0.20.1-3.fc33


Steps to Reproduce:
1. fedpkg clone python-MDAnalysis
2. fedpkg mockbuild


Actual results:
______ ERROR collecting MDAnalysisTests/visualization/test_streamlines.py ______
/usr/lib/python3.9/site-packages/py/_path/local.py:704: in pyimport
    __import__(modname)
__init__.py:125: in <module>
    matplotlib.use('agg', warn=False)
E   TypeError: use() got an unexpected keyword argument 'warn'


Expected results:
Tests pass without error.


Additional info:
You can fetch a patch for this from upstream:
https://github.com/MDAnalysis/mdanalysis/pull/2763