Number type hint

[Polandia94]

Issue: quantity is defined as Number, but all type checkers consider that int is not a subtype of Number.

Details:
In all tests and examples, quantity is an int, but it could also be a float or a Decimal (even though Decimal is not part of Number). Despite this, all three types would work fine.

Possible Solutions
We should decide on the expected behavior:

• Restrict quantity to int (probably the most common use case).
• Allow int and float.
• Allow int, float, and Decimal, with a specific error when an operation involves both Decimal and float.

Whichever option we choose, we should update the documentation to clarify what type quantity is expected to be.

Related Issues & References
python/mypy#3186
https://peps.python.org/pep-0484/#the-numeric-tower

[Brixt18]

I haven't really thought about the parameter to be a specific type of number (int, float, Decimal or other) but it should be, as it says, a number.
So, I would not restrict the type hint to int, so as the dev could use any kind of number they're working with, but I do think the 3rd approach you propose is the one we could take.

About the docs, the update will change the respective doc-string. And the readme (and other docs outside the code itself) will be updated soon as long I can work on it.