pythonPackages.mahotas: fix build, move expression

See https://hydra.nixos.org/build/80828287

Moves `mahotas` out of `python-packages.nix` into its own file and fixes
broken test cases by skipping them using nosetest's `@nottest`
annotation.

These tests broke from time to time in a sandbox and are therefore
considered impure.

Addresses #45960