feat: add mypy typing support

[dan-wilton]

Hey, nice implementation of snowflake IDs! 🙌
Thought I'd throw in some mypy support.

• Renamed the import of datetime to dt to fix datetime() property scope conflict
• Added mypy dependency with types-requests
• Enabled strict mypy rule
• Excluded tests folder from mypy type-checking
• Added py.typed file to indicate that typing is enabled on this project for consumers
• Replaced pytest.ini with equivalent pyproject.toml rules

If this is useful, I'm also happy to add in some pre-commit hooks and GitHub workflows?
May also be a good idea to have some linting in place as well - I'm happy to set up ruff in a separate PR if interested?

[vd2org]

Nice work! Please give me some more time to review.