Add type hints to pancake_sort (#14737)

[shreya-learn-explores]

Describe your change:

This pull request adds missing type hints to the pancake_sort function in sorts/pancake_sort.py.

Changes made:

• Added MutableSequence import from collections.abc
• Added a type hint to the arr parameter
• Added a return type annotation

This improves code readability and consistency with the repository's type hinting standards.

• 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 add type hints to pancake_sort function #14737".

[diusazzad]

Great job fixing those type hints! Really appreciate the effort.
Before we can merge this, can you make these quick tweaks to line up with our contributing guidelines?Put from future import annotations right at the top.Strip out the input() prompts from lines 38-42. We want to avoid manual inputs and stick to doctest.testmod().Optional but great to have: please add the Time (O(n^2)) and Space (O(n)) complexities inside the docstring.Thanks again, keep it up