Require matplotlib < 3.1 to fix gwpy incompatibility
Require matplotlib < 3.1 becuase matplotlib 3.2 breaks importing gwpy:
/usr/local/lib/python3.7/site-packages/gwpy/plot/rc.py:79: in <module>
rcParams.get('text.latex.preamble', []) + tex.MACROS),
E TypeError: can only concatenate str (not "list") to str