The package is currently broken and most folks recommend to use
`flask-caching` (see
thadeusb/flask-cache#171).

The only package in `nixpkgs` which remained to use `flask_cache`
(`graphite_api`) still builds with `pythonPackages.flask-caching`.

The removal and corresponding `graphite_api` change unbreaks several
Hydra builds (see https://hydra.nixos.org/build/76953777).

According to the current convention python packages should live in
`pkgs/development/python-modules`. As I altered the `graphite_api`
expression previously in d3d1b88 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`

Remove `pythonPackages.flask_cache`