Dependency on old matplotlib version

[MBradbury]

PyDTMC forces a dependency on an old matplotlib version (https://github.com/TommasoBelluzzo/PyDTMC/blob/master/setup.cfg#L60). This has knock on effects in terms of the numpy and scipy versions that can be installed.

It would be useful to either:

1. Remove the requirement on version 3.7.3 of matplotlib, or
2. Make matplotlib an optional dependency for users who do not use the plotting features of PyDTMC.