Drop sphinxcontrib-asyncio doc dependency

[musicinmybrain]

It is not compatible with current versions of Sphinx (aio-libs/sphinxcontrib-asyncio#15), and none of the extra syntax documented in https://sphinxcontrib-asyncio.readthedocs.io/en/latest/ appears to be used anyway.

What do these changes do?

Remove the sphinxcontrib-asyncio extension from the Sphinx configuration in docs/conf.py and from the requirements file requirements/doc.txt

Are there changes in behavior for the user?

No, the documentation still builds without warnings and looks normal.

Related issue number

N/A

Checklist

• I think the code is well written
• Unit tests for the changes exist N/A, nothing to test
• Documentation reflects the changes N/A, there doesn’t seem to be anything to document
• If you provide code modification, please add yourself to CONTRIBUTORS.txt N/A, a trivial non-code contribution
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• [x Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."