Move use of typing_extensions into TYPE_CHECKING block

[duncanmmacleod]

This PR moves all (one) usage of typing_extensions into a if TYPE_CHECKING block, so that the typing-extensions package is not required at runtime.

This might relate to #2281.

First-time contributor, sorry if I haven't followed the right contributing procedure.

[codecov]

Codecov Report

Attention: Patch coverage is 16.66667% with 5 lines in your changes missing coverage. Please review.

Project coverage is 81.55%. Comparing base (37d4205) to head (f00ee0c).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
kombu/asynchronous/semaphore.py	16.66%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2283      +/-   ##
==========================================
- Coverage   81.60%   81.55%   -0.06%     
==========================================
  Files          77       77              
  Lines        9540     9541       +1     
  Branches     1162     1162              
==========================================
- Hits         7785     7781       -4     
- Misses       1563     1568       +5     
  Partials      192      192              

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[Copilot]

Copilot reviewed 1 out of 3 changed files in this pull request and generated no comments.

Files not reviewed (2)

• requirements/default.txt: Language not supported
• requirements/pkgutils.txt: Language not supported

Comments suppressed due to low confidence (1)

kombu/asynchronous/semaphore.py:16

• Ensure that the alias 'P' is used exclusively for type annotations. If any runtime code relies on 'P', its definition being limited to the TYPE_CHECKING block might lead to a NameError.

P = ParamSpec("P")

[auvipy]

that's a good one!