Better type checking for XBlock fields

[kdmccormick]

Given a class like this:

class MyBlock(XBlock):
    my_bool = xblock.fields.Boolean(...)

I want mypy to understand this:

type(MyBlock.my_bool)               # --> xblock.fields.Boolean
type(instance_of_my_block.my_bool)  # --> bool

We can kinda hack around it right now by doing this:

class MyBlock(XBlock):
    my_bool: bool = xblock.fields.Boolean(...)

but that annotation isn't entirely correct, because the class-level attribute will always be an instance of xblock.fields.Field.

We should see what django-stubs does in order to make this work for Django models, which are in a similar situation.

[kdmccormick]

I'm folding this into:

• Add type hints and check them in CI #707

because I can't see how to add useful type hints to XBlock without making the fields behave correctly.

Fortunately, I think this should be doable, because XBlock fields are implemented using the descriptor protocol, which mypy is awesome enough to understand 🔥 I'm pretty sure that this is how Django does things, too.