feat: provide cv2.typing aliases at runtime

[VadimLevin]

Provide cv2.typing aliases at runtime.

• cv2.typing is generated as __init__.py instead of __init__.pyi
• built-in generics are changed to types from built-in typing module to handle Python versions < 3.9
• Type aliases comments are changed to doc strings. Example:

import typing

IntPointer = int
"""Represents an arbitrary pointer"""
MatLike = typing.Union[cv2.mat_wrapper.Mat, numpy.ndarray[typing.Any, numpy.dtype[numpy.generic]]]
MatShape = typing.Sequence[int]
Size = typing.Sequence[int]
"""Required length is 2"""

Resolves: #23782
Resolves: #23778

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
• The PR is proposed to the proper branch
• There is a reference to the original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake

[VadimLevin]

@Avasam take a look please

[Avasam]

I can't really test anything until at least tomorrow.
But from the PR description and a cursory glance at the changes this looks good.

[asmorkalov]

👍

[asmorkalov]

Tested manually with Ubuntu and Python 3.8.