Change matplotlib to >=3.1.3

[JacobHayes]

matplotlib 3.1.3 was released in Feb 2020 and does not provide wheels for newer python versions, making installation difficult. cellprofiler-core uses >=3.1.3 and other users seem to have no issues with newer versions, so this relaxes the constraint to >=3.1.3 here.

[ctr26]

Would be very helpful

[ergonyc]

This update would help me deploy a project leveraging centrosome. It's currently impossible to build the old matplotlib for recent pythons

[ctr26]

@bethac07 thoughts? Thanks

[ergonyc]

@bethac07 thoughts? Thanks

please consider 😃