Pygenetic no longer using importable version of matplotlib in python 3.8

[smithblack-0]

Hello, having a little trouble using your library so I will probably fork it.

Problem

Pygenetic is not using a version of matplotlib which can be installedin python 3.8
Version 2.2.2 of matplotlib is not available

To reproduce:

Make or access a python 3.8 install.
Attempt to install pygenetic, or even matplotlib 2.2.2
Get told it is not possible, as matplotlib 2.2.2 cannot be found.

I will open a pull request once I know updating to a newer version will not break anything.