np.ufunc type stubs (threading functions and PyUFunc_*) - #10739

A quick code search showed that these threading functions (documented in https://numba.readthedocs.io/en/stable/user/threading-layer.html) are used quite a lot, so I figured it'd help to stub them.

These threading functions actually live in numba.np.ufunc.parallel. But that module also contains a bunch of publically-named undocumented functions and classes. If I would've stubbed that module directly, then stubtest would've required me to stub everything there, which seemed like a waste of time.
These functions are re-exported in __init__.py , so it's a lot easier to place the stubs in this __init__.pyi. However, because this __init__.py also re-exports some constants from the C-extension module numba.np.ufunc._internal, I also had to stub these constants.

Anway, the stubs themselves are all very straightforward stuff.

Note that the vectorize/guvectorize re-exports here are not yet typed, see #10715. It's fine to merge this before, though.