Run sync context managers in a single thread

[adriangb]

This avoids a deadlock where __exit__ can't be called because there are no thread tokens available. Partially resolves #3205 and other related issues.

[adriangb]

cc @graingert @Kludex

[codecov]

Codecov Report

Merging #5114 (d6202f5) into master (8047230) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #5114   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          532       533    +1     
  Lines        13672     13716   +44     
=========================================
+ Hits         13672     13716   +44     

Impacted Files	Coverage Δ
fastapi/concurrency.py	100.00% <100.00%> (ø)
tests/test_concurrency.py	100.00% <100.00%> (ø)

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[github-actions]

📝 Docs preview for commit 732c0e9 at: https://62c8571bf399773d59da1a9b--fastapi.netlify.app

[github-actions]

📝 Docs preview for commit c2d73cd at: https://62c8582b4d7df6396e3dd6a6--fastapi.netlify.app

[github-actions]

📝 Docs preview for commit e39c97c at: https://62c866fbf77cbd4b372cd172--fastapi.netlify.app

[github-actions]

📝 Docs preview for commit d6202f5 at: https://62c8680e69e7074ec8565f20--fastapi.netlify.app

[adriangb]

Closing in favor of #5122