Add the type stubs and tests from numpy-stubs. Things this entails:
- Copy over the stubs (numpy/__init__.pyi and
numpy/core/_internal.pyi)
- The only modification made was removing `ndarray.tostring` since
it is deprecated
- Update some setup.py files to include pyi files
- Move the tests from numpy-stubs/tests into numpy/tests
- Skip them if mypy is not installed (planning on setting up CI in a
future PR)
- Add a mypy.ini; use it to configure mypy in the tests
- It tells mypy where to find NumPy in the test env
- It ignores internal NumPy type errors (since we only want to
consider errors from the tests cases)
- Some small edits were made to fix test cases that were emitting
deprecation warnings
- Add numpy/py.typed so that the types are picked up in an
installed version of NumPy