Re-enable keep-alive on proxies

[Dreamsorcerer]

trafficstars

Fixes #5765.

This was disabled a long time ago in #3070. From what I've seen of various issues, the sentiment from users is that this causes problems and makes proxies too slow. asvetlov himself has suggested that modern proxies today may not have the issues he was experiencing when the change was made originally.

The original change had no test and I'm not familiar enough with the proxy code to write a new test. I suggest we put this into 3.11 and see if any users report regressions.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.25%. Comparing base (1d21dce) to head (dbafc81). Report is 1083 commits behind head on master.

:white_check_mark: All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8920   +/-   ##
=======================================
  Coverage   98.25%   98.25%           
=======================================
  Files         107      107           
  Lines       34125    34138   +13     
  Branches     4048     4048           
=======================================
+ Hits        33530    33543   +13     
  Misses        421      421           
  Partials      174      174           

Flag	Coverage Δ
CI-GHA	98.15% <ø> (+<0.01%)	:arrow_up:
OS-Linux	97.81% <ø> (+<0.01%)	:arrow_up:
OS-Windows	96.21% <ø> (+<0.01%)	:arrow_up:
OS-macOS	97.48% <ø> (+<0.01%)	:arrow_up:
Py-3.10.11	97.58% <ø> (+<0.01%)	:arrow_up:
Py-3.10.14	97.51% <ø> (+<0.01%)	:arrow_up:
Py-3.11.9	97.74% <ø> (+<0.01%)	:arrow_up:
Py-3.12.4	?
Py-3.12.5	97.86% <ø> (+0.31%)	:arrow_up:
Py-3.9.13	97.47% <ø> (+<0.01%)	:arrow_up:
Py-3.9.19	97.41% <ø> (+<0.01%)	:arrow_up:
Py-pypy7.3.16	97.02% <ø> (+<0.01%)	:arrow_up:
VM-macos	97.48% <ø> (+<0.01%)	:arrow_up:
VM-ubuntu	97.81% <ø> (+<0.01%)	:arrow_up:
VM-windows	96.21% <ø> (+<0.01%)	:arrow_up:

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[patchback[bot]]

Backport to 3.11: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.11/5cf5db569c8f32b5ecd6a1586e4b8c349767d941/pr-8920

Backported as https://github.com/aio-libs/aiohttp/pull/8939

🤖 @patchback I'm built with octomachinery and my source is open — https://github.com/sanitizers/patchback-github-app.

[bdraco]

force_close is being called externally

https://github.com/aio-libs/aiodocker/issues/918#issuecomment-2486278493