gh-118761: Lazily import annotationlib in typing

[JelleZijlstra]

annotationlib is used quite a few times in typing.py, but I think the
usages are just rare enough that this makes sense.

The import would get triggered by:

• Using get_type_hints(), evaluate_forward_ref(), and similar introspection
  functions
• Using a string annotation anywhere that goes through _type_convert (e.g.,
  "Final['x']" will trigger an annotationlib import in order to access the
  ForwardRef class).
• Creating a TypedDict or NamedTuple (unless it's empty or PEP 563 is on).

Lots of programs will want to use typing without any of these, so the tradeoff
seems worth it.

• Issue: Improve import time of various stdlib modules #118761

[AlexWaygood]

I agree with the concept, but I think there's some opportunities to make this simpler and faster

[AlexWaygood]

the code doesn't look so pretty anymore, but I'm fine with this if it significantly speeds up the import time! (Do you have any numbers there?)

[JelleZijlstra]

On this branch with -X importtime:

import time:      2162 |       4424 | typing

On main:

import time:      2017 |       7641 | typing

[AlexWaygood]

Nice work, thank you!!