Use a distinct type for sentinel

[asvetlov]

Before sentinel was processed as either object or Any, both variants are far from perfectness.

Now sentinel has a dedicated type which is not equal to anything.

[codecov]

Codecov Report

Merging #5287 (57ba606) into master (7f73270) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #5287   +/-   ##
=======================================
  Coverage   97.52%   97.53%           
=======================================
  Files          43       43           
  Lines        8784     8787    +3     
  Branches     1410     1410           
=======================================
+ Hits         8567     8570    +3     
  Misses        103      103           
  Partials      114      114           

Flag	Coverage Δ
unit	97.41% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
aiohttp/connector.py	96.48% <100.00%> (ø)
aiohttp/helpers.py	96.78% <100.00%> (+<0.01%)	⬆️
aiohttp/payload.py	95.34% <100.00%> (+0.02%)	⬆️
aiohttp/test_utils.py	99.68% <100.00%> (ø)
aiohttp/web_request.py	97.61% <100.00%> (+<0.01%)	⬆️

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[asvetlov]

The missing part is the Ellipsis (...) handling.
I very much appreciate if somebody bears this change.
Sure, I can help with the review etc.

[aio-libs-github-bot]

💔 Backport was not successful

The PR was attempted backported to the following branches:

• ❌ 3.8: Commit could not be cherrypicked due to conflicts

[asvetlov]

Give up backpoting, should be ok as is.