Fix for race condition on connections in BaseConnector

[PandaWill]

What do these changes do?

Demonstrates & fixes race condition bug reported in #4936

Are there changes in behavior for the user?

None

Related issue number

#4936

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.
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  • if you don't have an issue_id change it to the pr id after creating the pr
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    • .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-commenter]

Codecov Report

Merging #4937 into master will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4937      +/-   ##
==========================================
+ Coverage   97.58%   97.61%   +0.03%     
==========================================
  Files          43       43              
  Lines        8932     8932              
  Branches     1406     1406              
==========================================
+ Hits         8716     8719       +3     
+ Misses         96       95       -1     
+ Partials      120      118       -2     

Impacted Files	Coverage Δ
aiohttp/connector.py	96.44% <100.00%> (+0.16%)	⬆️
aiohttp/web_fileresponse.py	97.82% <0.00%> (-0.55%)	⬇️
aiohttp/pytest_plugin.py	97.51% <0.00%> (+1.86%)	⬆️

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[asvetlov]

Thanks!