python3Packages.matplotlib: 3.1.3 -> 3.2.1, and various cleanups (#84362)

* pythonPackages.matplotlib: add veprbl to maintainers

* pythonPackages.matplotlib: remove an old darwin hack

* pythonPackages.matplotlib: remove python and stdenv from buildInputs

* python3Packages.matplotlib: remove a hack for python33

* pythonPackages.matplotlib: remove outdated checkPhase

Downloading the baseline_images is not a real issue, building against
older freetype (local_freetype = True) is, perhaps, not what we
want. The good news is that the tests would pass (tested on 3.2.1) if
we were to enable them:

========== 6684 passed, 1332 skipped, 10 xfailed in 228.64s (0:03:48) ==========

* pythonPackages.matplotlib: provide setup.cfg from a file instead of a patch

* python3Packages.matplotlib: 3.1.3 -> 3.2.1