[typing] use less restrictive type hints for some public methodes

[neoniobium]

I when using cmd2, I found the type hints to be to restrictive since I like to use tuples and frozensets.

Hence ,I changed some list types to Iterable or Sequence depending if len seamed to be required.

No functional change intended.

Please review, and feel free to discard.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.94%. Comparing base (781f613) to head (9132109).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1547   +/-   ##
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  Coverage   98.94%   98.94%           
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  Files          21       21           
  Lines        4942     4942           
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  Hits         4890     4890           
  Misses         52       52           

Flag	Coverage Δ
unittests	98.94% <100.00%> (ø)

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[kmvanbrunt]

Thanks for this suggestion. While I'm not ready to accept this exact change just yet, I do agree with its intentions. cmd2 is about to go through a major re-architecture as we port to prompt_toolkit. During that process I will be changing many of these return types. In fact, I have already started using Sequence in our new code.

I have added this to our #1538 cmd2 4.0 task list.

[tleonhardt]

We revisited the intent of this PR in #1584 . Changes will be released in version 4.0.0.