Delay evaluation of type hint imports

[paxnovem]

This PR reduces the time at startup that is taken at import time. This is achieved by delaying the evaluation of type imports.

• investigate and improve startup speed #2267

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[saroad2]

Since this PR was written, the PSF has decided to go on a different direction than from __future__ import annotations. You can read about it here. Right now they consider moving towards PEP 649 instead.

Unfortunately, I think we might should drop this PR. @davidism , would love to hear your opinion on it.

[davidism]

from __future__ import annotations will continue to work as it does, and any change will still result in deferred evaluation of annotations.

[davidism]

Updated to use modern typing syntax and features. Both MyPy and PyRight support using modern syntax with older Pythons when using from __future__ import annotations.

• Remove typing_extensions where possible.
• Use shorthand union syntax | instead of Union and Optional.
• Use generic types from built-ins, collections, collections.abcandcontextlib`.
• Quote TypeVar bound and cast types to avoid runtime evaluation of generics and unions.