Stop testing on python 3.3 (and remove mentions of 3.3 support from docs etc)

[ceball]

The "pip/python build toolchain" no longer supports python 3.3; the current tests on travis error out with wheel requires Python '>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*' but the running Python is 3.3.6

Although it would still be possible to keep testing already built packages in a python 3.3 environment (i.e. build package with e.g. python 3.6, then test on 3.3), I'm not sure it's worth the effort.

3.3 end of life was September 2017