Add missing section comment to __all__ definition of aiohttp

[AMDmi3]

What do these changes do?

__all__ entries in aiohttp are diveded into sections in accordance of what subpackage they come from by comments. One comment for streams is missing, add it.

Are there changes in behavior for the user?

No

Related issue number

None

Checklist

• I think the code is well written
• Unit tests for the changes exist (cosmetic change, not needed)
• Documentation reflects the changes (cosmetic change, not needed)
• If you provide code modification, please add yourself to CONTRIBUTORS.txt (done in Use correct style for re-exported imports #3872)
• Add a new news fragment into the CHANGES folder (cosmetic change, not needed)

[codecov-io]

Codecov Report

Merging #3874 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3874   +/-   ##
=======================================
  Coverage   97.93%   97.93%           
=======================================
  Files          43       43           
  Lines        8590     8590           
  Branches     1373     1373           
=======================================
  Hits         8413     8413           
  Misses         72       72           
  Partials      105      105

Impacted Files	Coverage Δ
aiohttp/__init__.py	100% <ø> (ø)	⬆️

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