Remove Python 2 from package classifiers

[dopplershift]

We already say requires_python = >=3.6

[ConorMacBride]

The readme also mentions Python 2.7 . Also, should the minimum Python 3 version be kept in sync with matplotlib/matplotlib, or just increased when necessary?

[dopplershift]

Fixed the readme. I don't think there's a need to stay in lockstep with matplotlib, with the exception that our minimum Python and minimum matplotlib have to make sense.

[Cadair]

Thanks!