gh-101100: Fix sphinx warnings in concurrent.futures.rst

[sobolevn]

Errors fixed:

/Users/sobolev/Desktop/cpython2/Doc/library/concurrent.futures.rst:277: WARNING: py:exc reference target not found: BrokenProcessPool /Users/sobolev/Desktop/cpython2/Doc/library/concurrent.futures.rst:502: WARNING: py:const reference target not found: FIRST_COMPLETED /Users/sobolev/Desktop/cpython2/Doc/library/concurrent.futures.rst:505: WARNING: py:const reference target not found: FIRST_EXCEPTION /Users/sobolev/Desktop/cpython2/Doc/library/concurrent.futures.rst:505: WARNING: py:const reference target not found: ALL_COMPLETED /Users/sobolev/Desktop/cpython2/Doc/library/concurrent.futures.rst:511: WARNING: py:const reference target not found: ALL_COMPLETED /Users/sobolev/Desktop/cpython2/Doc/library/concurrent.futures.rst:572: WARNING: py:class reference target not found: ThreadPoolExecutor /Users/sobolev/Desktop/cpython2/Doc/library/concurrent.futures.rst:582: WARNING: py:class reference target not found: ProcessPoolExecutor 

Refs #114469

• Issue: Fix all Sphinx reference warnings in the documentation #101100

---

📚 Documentation preview 📚: https://cpython-previews--114521.org.readthedocs.build/

[hugovk]

Thanks! 🧹 📖

[miss-islington-app]

Thanks @sobolevn for the PR, and @hugovk for merging it 🌮🎉.. I'm working now to backport this PR to: 3.11, 3.12.
🐍🍒⛏🤖

[miss-islington-app]

Sorry, @sobolevn and @hugovk, I could not cleanly backport this to 3.12 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 8744ecf5896ccf57875574a9aed46369b8d48dc1 3.12 

[miss-islington-app]

Sorry, @sobolevn and @hugovk, I could not cleanly backport this to 3.11 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 8744ecf5896ccf57875574a9aed46369b8d48dc1 3.11 

[bedevere-app]

GH-114525 is a backport of this pull request to the 3.12 branch.

[bedevere-app]

GH-114526 is a backport of this pull request to the 3.11 branch.