Remove duplication of dtype/astype arguments between `_typing.pyi` and `tests/__init__.py`

[Dr-Irv]

Here's a possible way we can handle this.

What if we put all these declarations in a file _typing_dtypes.pyi, and _typing.pyi imports that file.

Then when we run the tests, we copy _typing_dtypes.pyi to a file tests/typing_dtypes.py and then import that file here.

So then we have exactly one copy. We just have to engineer the tests to do the copying first.

Originally posted by @Dr-Irv in #1493 (comment)