Fix Matplotlib type error for tests (#4023)

* Fix for issue 4022.

Check for support for `warn` argument in `matplotlib.use()` call. Drop it if it causes an error.

* Alternative fix.

pymc3/tests/test_examples.py

@@ -20,10 +20,14 @@
 import pytest
 import theano
 from pymc3.theanof import floatX
+from packaging import version
 
 from .helpers import SeededTest
 
-matplotlib.use('Agg', warn=False)
+if version.parse(matplotlib.__version__) < version.parse('3.3'):
+    matplotlib.use('Agg', warn=False)
+else:
+    matplotlib.use('Agg')
 
 
 def get_city_data():