Drop pytest-mpl

Since matplotlib have its own testing decorators, it may make sense to just change to those:

https://matplotlib.org/stable/api/testing_api.html#module-matplotlib.testing.decorators

In this way, the image tests will always be performed (no need for the --mpl switch, there is no need for the additional dependency on pytest-mpl, and the alternative images are directly generated.

@hugwi268 considering your image test failures.