According to the current convention python packages should live in
`pkgs/development/python-modules`. As I altered the `graphite_api`
expression previously in d3d1b88873 I decided to move it as well.
Additionally I applied some minor refactorings:
* use `fetchFromGitHub` instead of `fetchgit`.
* use `checkInputs` for test dependencies
* got rid of fixed points
* applied patch which supports flask-caching to 1.3.1 using `git apply
-3`
