support typing.Final annotation

[carljm]

Doc: https://docs.python.org/3/library/typing.html#typing.Final
Spec: https://typing.python.org/en/latest/spec/qualifiers.html#uppercase-final

• Infer the type of the RHS for public uses of bare Final symbols
• Add a new diagnostic for modifications of Final-qualified symbols
• Make sure that both of the above also works for attributes
• Prevent overriding of Final attributes in subclasses
• Report an error if there is no binding to a Final-qualified symbol
• Disallow Final in function parameter definitions
• Variance inference: if an attribute is marked Final, its type should be covariant rather than invariant

Somewhat related tickets: #433, #529