[mypy] Fix mypy error in data_structures/linked_list/singly_linked_list.py

[shermanhui]

Fix mypy error in data_structures/linked_list/singly_linked_list.py

The list comprehension shortcut was implicitly expecting a return
value causing a mypy error since insert_tail doesn't return a value.

This PR is my second contribution to fixing the mypy errors identified in #4052

• 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}.

[poyea]

Thank you for your pull request!🤩