Adding schema for typing.Final

[giograno]

While looking at the StepFunctions provider, I realized that our framework does not handle the typing.Final annotation.

For instance, for the following code:

class MyType:
   var: typing.Final[str]

the schema generation fails since there is no handler for Final.

We are fixing this by introducing a schema for typing.Final that simply strips out the subscribed type invoke that specific schema.

Closing PNX-474

[bentsku]

LGTM, nice catch!