Allow Final attributes declared in class body if initialized in ctor

There is currently a safety check around TypeVar-dependent Final attributes declared in the class body that I believe we can loosen when the variable is assigned in the ctor. This should work analogously to non-TypeVar-dependent final attributes declared in the class body, which also require an assignment in the ctor (otherwise they emit "Final name must be initialized with a value").

• Are you reporting a bug, or opening a feature request? feature request
• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

  from typing import Final, Generic, TypeVar
  
  
  WidgetT = TypeVar('WidgetT', bound='Widget')
  
  
  class Widget:
      pass
  
  
  class WidgetHolder(Generic[WidgetT]):
  
      widget: Final[WidgetT]
  
      def __init__(self, widget: WidgetT) -> None:
          self.widget = widget
  

• What is the actual behavior/output?

  final.py: note: In class "WidgetHolder":
  final.py:13:5: error: Final name declared in class body cannot depend on type variables  [misc]
          widget: Final[WidgetT]
          ^
  

In case anyone comes across this issue, the workaround is currently to not declare in the class body, but declare in the ctor with self.widget: Final[WidgetT] = widget instead. But just because a workaround exists doesn't mean we shouldn't try to fix this.

Start by running mypy on the provided final.py reproduction and inspect the existing checks for Final attributes declared in a class body, comparing them with non-TypeVar-dependent Final attributes. Done means the reproduction type-checks when the attribute is assigned in init, while the existing error remains for declarations that are not initialized there.