Throw NotImplementedError for HELLO

[dvora-h]

Pull Request check-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)?

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 #1912 (c1c5eec) into master (f9d8091) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1912   +/-   ##
=======================================
  Coverage   93.03%   93.03%           
=======================================
  Files          76       76           
  Lines       16567    16573    +6     
=======================================
+ Hits        15413    15419    +6     
  Misses       1154     1154           

Impacted Files	Coverage Δ
redis/commands/core.py	90.26% <100.00%> (+0.01%)	⬆️
tests/test_commands.py	99.40% <100.00%> (+<0.01%)	⬆️

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