ENH: Add annotations for five array construction functions

[BvB93]

This pull request adds annotations to the five array construction functions in np.core._asarray:

• asarray
• asanyarray
• ascontiguousarray
• asfortranarray
• require

[BvB93]

def(a: _ArrayType, ...) -> _ArrayType implies that both the array (sub-)class, dtype and shape are identical for the input and output.

Without higher-kindred typevars (python/typing#548) we, unfortunately, can not describe a situation were an ndarray subclass is returned with a dtype that is different from the initial input (hence the dtype: None in this overload).

[BvB93]

Rebased to get rid of a merge conflict.

[BvB93]

71def2c implements an alternative approach for ensuring that externally-defined stubs are
picked up in the main __init__.pyi file. Using from x import y as y is more compact than
specifying them in __all__ and, more importantly, it is less prone to merge conflicts.

[BvB93]

be77442 removes the placeholder annotations (see #17406) for the newly annotated functions introduced in this PR.

Secondly, if there are no further comments then I feel this can get merged.

[charris]

Thanks @BvB93 .