Update rod_cutting.py

[SandersLin]

A hopefully clearer implementation without dependence on global variables.

[cclauss]

Nice! Please consider adding a doctest.

[SandersLin]

Thanks! Added doctests.

[cclauss]

Doctests for length = -1, length = 3.2, prices = []

[SandersLin]

Modified doctests and type hints.
I wonder when doing input validation is too much though. For example, when the type hint and documentation clearly states prices should be List[int], but some bloke passed in list of strings, the author shouldn't be responsible.

[cclauss]

Really well done. I like the very Pythonic way that you raise exceptions on bad user input. Never assume that the end user has seen the type hints.