remove pytest.warns(None)

[bnavigator]

What do these changes do?

Fix a deprecated misuse of pytest.warns()

https://docs.pytest.org/en/latest/deprecations.html#using-pytest-warns-none

Since the test suire errors on any unfiltered warning anyway, these context manageres are not necessary.

Are there changes in behavior for the user?

No. Just the test suite

Related issue number

aio-libs#6663

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• 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."