Added doctest to heap.py

[Suyashd999]

Describe your change:

Contributes to #9943

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

[Suyashd999]

Hey @cclauss , @tianyizheng02 can you please review this PR I am unable to make more contributions as I have reached the limit of 5 open PR

[cclauss]

Let's test with zero, negative integers, floating point numbers, and strings.

Let's test h.parent_index(12) which is NOT in the heap.

We want to test how the algorithm works but also how it fails when given bad input.

[Suyashd999]

@cclauss done with the changes

[Suyashd999]

hey @cclauss can you review this please. Thanks!

[cclauss]

These tests prove that parent heap is buggy. It returns None when it should be raising a ValueError and it returns float when its type hints promised that it would only return int | None. These bugs should be fixed in a future pull request.