Stubs in `core/numerictypes.pyi` are not picked up

[person142]

This is the same issue as #50, but for core/numerictypes.pyi. The stubs are not imported into __init__.pyi, so they are not picked up. A quick test of that:

import numpy as np

reveal_type(np.maximum_sctype(np.dtype(np.float64)))

gives Revealed type is 'Any' on master. We should move those stubs into the top-level __init__.pyi and add tests.