buildPythonPackage: add shell hook for instant development
pythonPackages: syntax improvements

- import pkgs.lib by default, there's no reason why not
- Define `isPy*` helper functions for all python versions
- Define `pythonName` that returns unique python name based on `isPy*`
  helpers
- Fix `python2.7` version detection
- Fix pythonPackages importing(with) for `python2.7` only packages