[PR #7333/3a54d378 backport][3.8] Fix TLS transport is None error

[patchback]

This is a backport of PR #7333 as merged into master (3a54d37).

Fixes #3355

The type has been fixed in typeshed as well, so the bug here would be highlighted in mypy 1.5+.
python/typeshed#10346 (comment)

[codecov]

Codecov Report

Merging #7357 (dbd76ab) into 3.8 (4087410) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##              3.8    #7357      +/-   ##
==========================================
- Coverage   97.39%   97.39%   -0.01%     
==========================================
  Files         107      107              
  Lines       30996    30999       +3     
  Branches     3924     3925       +1     
==========================================
+ Hits        30188    30190       +2     
- Misses        604      605       +1     
  Partials      204      204              

Flag	Coverage Δ
CI-GHA	97.25% <0.00%> (-0.01%)	⬇️
OS-Linux	96.89% <0.00%> (-0.01%)	⬇️
OS-Windows	95.22% <0.00%> (-0.02%)	⬇️
OS-macOS	96.64% <0.00%> (+<0.01%)	⬆️
Py-3.10.11	94.83% <0.00%> (-0.02%)	⬇️
Py-3.10.12	96.53% <0.00%> (+<0.01%)	⬆️
Py-3.11.0	96.20% <0.00%> (-0.01%)	⬇️
Py-3.6.15	96.37% <0.00%> (+<0.01%)	⬆️
Py-3.6.8	94.75% <0.00%> (-0.02%)	⬇️
Py-3.7.17	96.52% <0.00%> (-0.01%)	⬇️
Py-3.7.9	94.88% <0.00%> (-0.01%)	⬇️
Py-3.8.10	94.80% <0.00%> (-0.01%)	⬇️
Py-3.8.17	96.48% <0.00%> (+0.04%)	⬆️
Py-3.9.13	94.81% <0.00%> (-0.01%)	⬇️
Py-3.9.17	96.45% <0.00%> (-0.01%)	⬇️
Py-pypy7.3.11	93.94% <0.00%> (-0.01%)	⬇️
VM-macos-latest	96.64% <0.00%> (+<0.01%)	⬆️
VM-ubuntu-20.04	96.88% <0.00%> (-0.01%)	⬇️
VM-ubuntu-latest	96.42% <0.00%> (-0.01%)	⬇️
VM-windows-latest	95.22% <0.00%> (-0.02%)	⬇️

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

Impacted Files	Coverage Δ
aiohttp/connector.py	93.26% <0.00%> (-0.40%)	⬇️

... and 1 file with indirect coverage changes

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