Remove support for Python 2.7

[tobinus]

Goes out of support January 1st 2020. Some may provide security fixes beyond that, but I think it is time to let it go. I don't expect this to be a big problem, since PodGen was published long after Python 3 was introduced.

Dropping support for a Python version is a breaking change I think (though pip will make sure incompatible versions are not installed – but only on newer pip versions). So it should at least wait until v2.0.