pythonPackages.mrbob: fix build

Tests broke on Hydra as the `checkPhase` wasn't configured properly. By
explicitly relying on `nosetests` and injecting `LC_ALL` into the
`checkPhase` the tests work again.

The license (bsd3) according to `LICENSE` distributed with the upstream
package wasn't specified in the meta section which could've caused legal
issues.

The expression has been moved into its own file to reduce the length and
complexity of `pkgs/top-level/python-packages.nix`.

See https://hydra.nixos.org/build/70689499/log
See #36453