Now thetests/subtests can be updated/regenerated automatically without needing to manually adjust the baseline data.

To create the baseline images and hashes run:

MPL_UPDATE_BASELINE=1 tox -e mpl35

Then, to create the baseline summaries run:

MPL_UPDATE_SUMMARY=1 tox -e mpl35

Finally check thegit diff and run the standard tests before committing:

tox -e mpl35

I will add documentation for this in a README file intests/subtests.

All changes are isolated to withintests/subtests except I've addedPillow (imported asPIL) as a dependency. (Matplotlib already requires this package and#150 will need it also.) And I have made some minor changes totox.ini.

Interestingly, removing the matplotlib version number from the PNGs means thatall the hashes are identical across matplotlib 3.3, 3.4 and 3.5!! (I guess this isn't too surprising since the plots are very basic.)

• Add documentation

Closes#159