Fix AttributeError in Explore CoDEx.ipynb

[1Maxnet1]

When following the Explore CoDEx.ipynb step by step at cell 15 with an AttributeError, as a certain method was deprecated in matplotlib 3.2 and later removed (see e.g. here https://github.com/pysal/splot/issues/115)

So to fix this I added a fix for the version in the requirements.txt. This makes the fixes the problem for me.

If you need any further information let me know :)