[PR #4912/9a599e92 backport][3.7] Fix the tracing module's type hints

[patchback]

This is a backport of PR #4912 as merged into master (9a599e9).

What do these changes do?

_SignalArgs is (was) a supertype of its
arguments and with Callable being contravariant
in its arguments, the only way this would've
passed a type check is if the callback were to
(purport to) accept every single one of the
parameter types in a union,
exactly as specified - or object.
Either way, the parameter types were not
associated with their corresponding signals
so it would have been possible to append
a callback with a TraceDnsCacheMissParams
parameter to an on_request_start signal.

Are there changes in behavior for the user?

No.

Related issue number

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Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[codecov-io]

Codecov Report

Merging #5038 into 3.7 will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              3.7    #5038      +/-   ##
==========================================
- Coverage   97.73%   97.73%   -0.01%     
==========================================
  Files          44       44              
  Lines        8953     8952       -1     
  Branches     1407     1407              
==========================================
- Hits         8750     8749       -1     
  Misses         87       87              
  Partials      116      116              

Flag	Coverage Δ
#unit	97.73% <100.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
aiohttp/tracing.py	100.00% <100.00%> (ø)

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