Add unit tests for flatten and iflatten in utils.py

[IkramKhanNiazi]

• Added tests for flatten with nested iterables including lists, tuples, sets, and generators.
• Added tests for iflatten to ensure it correctly returns an iterator over nested iterables.

[IkramKhanNiazi]

@wRAR, could you please review this PR?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.18%. Comparing base (fd183dd) to head (fc6ad79).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #331   +/-   ##
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  Coverage   95.18%   95.18%           
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  Files           5        5           
  Lines         436      436           
  Branches       76       76           
=======================================
  Hits          415      415           
  Misses         13       13           
  Partials        8        8           

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[wRAR]

It may be more simple to just add a doctest to iflatten().