Fix data_structures/disjoint_set/disjoint_set.py

[Alperencode]

Fixes: #4764

Describe your change:

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

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• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
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• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
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[Alperencode]

Its actually not a hard change, i just change the if statement to work more properly and close the issue-#4764

[poyea]

Please fix the failing test.

FAILED data_structures/disjoint_set/disjoint_set.py::data_structures.disjoint_set.disjoint_set.test_disjoint_set

I assume this change to be the correct one. Is there anything wrong in the current tests?

[Alperencode]

I did not test it and actually i dont know how to test. I just fixed the code to close issue #4768. And that was not that complex i just changed the a few lines that shouldn't cause any problem.

[poyea]

@Alperencode It says something wrong with https://github.com/TheAlgorithms/Python/blob/master/data_structures/disjoint_set/disjoint_set.py#L57. Take an overall look and see what's wrong with the test? or you may change those 2 functions (+find_python_set) into one better function.