Fixed type annotations for keys of multidict mapping classes

[q0w]

ref:
python/mypy#8477

What do these changes do?

Are there changes in behavior for the user?

Related issue number

Fixes #644

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> (e.g. 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.

[webknjaz]

Could you fill out the PR title/description?

[codecov]

Codecov Report

Merging #677 (4613144) into master (1322f1a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #677   +/-   ##
=======================================
  Coverage   93.77%   93.77%           
=======================================
  Files           5        5           
  Lines         498      498           
=======================================
  Hits          467      467           
  Misses         31       31           

Flag	Coverage Δ
unit	93.77% <ø> (ø)

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[asvetlov]

Thanks!