Support for Python 3.12

[ekzhu]

Why are these changes needed?

Support for Python 3.12.

Related issue number

#101 aiohttp now supports 3.12.

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (73c7522) 32.06% compared to head (2e32a8c) 41.88%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1317      +/-   ##
==========================================
+ Coverage   32.06%   41.88%   +9.81%     
==========================================
  Files          33       33              
  Lines        4422     4422              
  Branches     1034     1089      +55     
==========================================
+ Hits         1418     1852     +434     
+ Misses       2887     2421     -466     
- Partials      117      149      +32     

Flag	Coverage Δ
unittests	41.81% <ø> (+9.79%)	⬆️

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[sonichi]

The test passed. We can change the pull_request_target back. I suggest changing the banner in README.md and remove unnecessary newly added imports. Then I'll approve.

[ekzhu]

@sonichi Done.