Part of fix for #3481.

Updated type hinting in ignite/handlers/checkpoint.py.

Description:

• Replace Union[X, Y] and Optional[X] with X | Y and X | None
• Changed List -> list and Dict -> dict
• Remove unused imports

All tests passed.

Check list:

• New tests are added (if a new feature is added)
• New doc strings: description and/or example code are in RST format
• Documentation is updated (if required)

Please let me know if any changes are required or if I have missed anything.

Thanks!