m.plots: fix a deprecation warning with matplotlib 3.5.
m.plots: fix a deprecation warning with matplotlib 3.5.

It complains that "Support for setting an rcParam that expects a str
value to a non-str value is deprecated since 3.5 and support will be
removed two minor releases later."

mosra/m.css,mosra/m.css,mosra/m.css,mosra/m.css