Feature request: add bool conversion to `Maybe`

[InAnYan]

Refs: #874.

Currently, Maybe type doesn't have special methods for being treated as bools, so any Maybe object is True in Python.

However, I find that it doesn't fit well in Python idioms. One cool feature of Optional in Python is that I can do statements like this:

if await self.find_model(id):
    <do something>
...

So I can easily check if the value is truthy or falsey. I don't need to write not is None.

However, if self.find_model would return a Maybe type, then code becomes more complex:

if (await self.find_model(id)) != Nothing:
    <do something>
...

I believe it would be a good addition to Maybe to add __bool__ method.

Couple of remarks:

1. I'm aware that checking whether the object is None and falsiness of a value in Python are different things. Any Python developer should be aware of this. And if anyone would return a list or strings in find_model, then that person wouldn't really be using Optional or Maybe, I think.
2. One of the maintainer raised a question in other issue (Request: Truthiness of Result, Correct way to filter over Iterable[Result] #874 (comment)):

It is not clear what we check with bool(), the container itself? Or its content? In other words: Success(False) is True or False?

In my opinion, it's crystal clear. If I analyze a Maybe object then I analyze Maybe object, not it's value. It aligns with the documentation as well (https://returns.readthedocs.io/en/latest/pages/maybe.html#how-to-model-absence-of-value-vs-presence-of-none-value):

 So, the first thing to remember is that:

>>> assert Some(None) != Nothing

I have one more positive argument to this semantic decision: imagine one would like to model these kinds of value:

1. Undefined (not supplied, not present).
2. None (present, but null).
3. Present.

This could be easily modeled if __bool__ would check whether a Maybe is Nothing or Something:

1. Undefined - Nothing.
2. None - Something(None).
3. Present - Something(123).

[sobolevn]

Ok, let's add __bool__ -> False to Nothing and __bool__ -> True to Some :)

[sobolevn]

Would you like to send a PR? :)

[InAnYan]

Would love, if I have time

[kodsurfer]

+1

[sobolevn]

@kodsurfer do you want this feature? Or do you want to work on it? :)