Fix typing on smembers command

[agusdmb]

Pull Request check-list

The smembers command returns a set, but it is typed as returning a list

Please make sure to review and check all of these items:

• Does $ tox pass with this change (including linting)?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?
• Was the change added to CHANGES file?

NOTE: these things are not required to open a PR and can be done
afterwards / while the PR is open.

Description of change

Please provide a description of the change here.

[codecov-commenter]

Codecov Report

Merging #2312 (c749af4) into master (c94821c) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2312   +/-   ##
=======================================
  Coverage   92.16%   92.16%           
=======================================
  Files         110      110           
  Lines       28642    28642           
=======================================
  Hits        26399    26399           
  Misses       2243     2243           

Impacted Files	Coverage Δ
redis/commands/core.py	82.11% <100.00%> (ø)
tests/test_cluster.py	96.96% <0.00%> (ø)

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