nitypes: Delete _typing submodule and use typing_extensions directly

[bkeryan]

• This contribution adheres to CONTRIBUTING.md.

What does this Pull Request accomplish?

Add a dependency on the typing-extensions package and import symbols directly from typing_extensions.

Why should this Pull Request be merged?

Single-sourcing typing_extensions imports in a private _typing submodule is not working well. I'm starting to think that avoiding typing_extensions at run time is not worth it. It is lightweight and has no external dependencies.

With a private _typing submodule, sphinx-autoapi renders type aliases as nitypes._typing.TypeAlias. If we use typing.TypeAlias directly, it renders them as a PEP 695 type statement showing what they are aliased to. I have proposed updating sphinx-autoapi to do the same for typing_extensions.TypeAlias: readthedocs/sphinx-autoapi#521

This pattern works for sphinx-autoapi, but not for mypy, due to python/mypy#14220

try:
    from typing import TypeAlias
except ImportError:
    from typing_extensions import TypeAlias  # or from nitypes._typing

What testing has been done?

Ran nps, mypy, pytest.
Generated docs with readthedocs/sphinx-autoapi#521 and verified that type aliases are rendered properly.

[github-actions]

Test Results

   10 files  ±0     10 suites  ±0   25s ⏱️ -1s
  511 tests ±0    511 ✅ ±0  0 💤 ±0  0 ❌ ±0 
5 110 runs  ±0  5 110 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit ec163ae. ± Comparison against base commit 10bdfbe.