MAINT: handle not implemented arguments centrally, via a dedicated annotation

[ev-br]

as discussed in #112 (comment)

This does seem to gain a marginal LOC reduction and some possible readability improvement. Two points:

• I'm {ab,re}using the NotImplemented singleton as the annotation. Introducing a new NotImplementedType is a bit awkward since typing.NotImplementedType is a thing from python 3.10, and we need to cover 3.8+. A simple alternative is to just use type(NotImplemented) instead of NotImplemented. Don't have a firm opinion, am open to suggestions.

• NumPy uses a weird _NoValue sentinel in some cases; we mimicked it by defining an alias NoValue = None. In our usage we typically raise NotImplementedError on things with the NoValue default. Now that we have a dedicated annotation, NoValue alias seems extraneous. Anyone sees a need for it?

[lezcano]

I really like this! It really shows that, either you should be processing an arg in the implementation, or you should mark it as NotImplemented, so there's no ambiguity for "did you forget to use this arg, or is it just not implemented?"

About the _NoValue, it makes sense to remove it and simply replace it by None.

About the NotImplementedType, I'd say it's best to have the usual pattern of

try:
  from typing import NotImplementedType
except ImportError:
  NotImplementedType = typing.TypeVar("NotImplementedType")

The reason is that NotImplemented is an object, not a class, so using it for typing is a bit odd. WDYT?

[lezcano]

Great, thank you!