Refactor character filtering for consistency in pangram check

[Tithi234]

Describe your change:

• Simplifies character filtering logic in is_pangram
• Uses str.isalpha() for clarity and consistency
• Keeps behavior and doctests unchanged

What type of change is this?

• Fix a bug or typo in an existing algorithm?

Checklist:

• I have read CONTRIBUTING.md
• This pull request is all my own work
• I know that pull requests will not be merged if they fail the automated tests
• This PR only changes one algorithm file
• 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

[Tithi234]

Closing this PR to keep the repository clean as the current CI checks are failing.
I’ll revisit this with a cleaner approach in the future.
Thanks to the maintainers for the review process.