Valkyrie-util

fix typed declaration

add py.typed so package comes with type hints for mypy

added mypy type checking and improved performance

New methods

* factors.proper_divisor_sum(n: int) -> int

New methods

* factors.proper_divisors(n: int) -> Set[int]

New methods

* collatz.next_collatz(n: int) -> int
* collatz.collatz_iterator(n: int) -> Generator[int, None, None]
* collatz.collatz_length(n: int) -> int