Fix handling of non ascii characters in swap case (fixes: #3847)

[frankschmitt]

Describe your change:

This fixes 3847 - handling of non-ASCII characters in swap_case.

Output before (buggy):
python3 swap_case.py
Please input sentence:äü
ÄäÜü

Output now (correct):
python3 swap_case.py
Please input sentence:äü
ÄÜ

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• 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 have a URL in its comments that points to Wikipedia or other similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.
  => It's not contained in the commit messages, but in the PR description. I hope this will suffice. Otherwise, I can of course add a dummy commit.

[frankschmitt]

I noticed that the pre-commit hook failed - apparently due to some reformatting?

Excerpt from the log:

black....................................................................Failed

• hook id: black
• duration: 10.08s
• files were modified by this hook

reformatted /home/runner/work/Python/Python/strings/swap_case.py
All done! ✨ 🍰 ✨
1 file reformatted, 823 files left unchanged.

Is there any action required by me to fix this?

[mrmaxguns]

Would be good if type hints were added to the function, i.e.:

from typing import Text

def swap_case(sentence: Text) -> Text:
    ...

[dhruvmanila]

Thank you for fixing this issue!