Adding missing return type to pi_estimator function (#13427)

- Add -> None return type annotation to pi_estimator function
- Improves code clarity and follows Python type hinting best practices
- Function already had proper type hints for parameters

Co-authored-by: Gunish Mukherji <gunishmukherji@Gunishs-MacBook-Air.local>
Co-authored-by: Maxim Smolskiy <mithridatus@mail.ru>