This adds another set of type annotations for #12

• Mypy passes the adafruit_itertools module without errors.
• Added tests for functionality and to exercise type annotations, and mypy also passes these tests without errors.

I've added a few #type: ignores in the library where the code uses sentinels or just to simplify things.

Additional notes for annotations that could be worth a second look:

accumulate: can theoretically accumulate a different type than the input list, but can be somewhat surprising as it only starts from the second element. e.g. list(it.accumulate([1, 2], lambda x, y: str(x) + str(y))) returns [1, "12"]. I've chosen to restrict it so the accumulator also produces the same type as the iterator, as it seemed more likely to catch unintended errors, but there are arguably other reasonable choices.

I made the opposite decision in a few places by using Any to keep things simple, and/or there isn't enough support in micropython (class generics, @overload etc).

groupby: is implemented as a class, but unfortunately it proved difficult to annotate the class itself (or the __init__ method) usefully. So I kept it simple, but relaxed; but callers may need to annotate the result to get useful type checks.

product, zip_longest: relaxed the type checking on the incoming iterators as it seemed more likely that callers would
intentionally use different types in the iterators, and it was simpler to relax the constraints.

(I'll repeat the process for the adafruit_itertools_extra once this batch of annotations gets in.)