They're all redundant on Python 3

and fix it

Replace versioneer with hand-rolled version generation code.

also refactor tests.nix and default.nix so they share more of the environment
setup which is now slightly more complex

the branch has some python packaging fixes in nixpkgs that we need

there will be tons of pyflakes warnings from the python3 porting strategy

Refactor the Nix packaging for reduced duplication and to provide a more useful shell environment

it is nice to fail if someone passes in something that is not supposed to be
passsed in, rather than silently accepting and ignoring it