docs: Improve "Precise typing of alternative constructors" example

See microsoft/pyright#4001.

- TypeVar bounds that contain TypeVars are not well-specified and pyright disallows them
- Similarly, TypeVars defined in classes raise weird scoping problems

Let's keep the examples portable and avoid relying on mypy-specific features.

docs/source/more_types.rst

@@ -804,10 +804,9 @@ classes are generic, self-type allows giving them precise signatures:
 .. code-block:: python
 
    T = TypeVar('T')
+   Q = TypeVar('Q', bound='Base[Any]')
 
    class Base(Generic[T]):
-       Q = TypeVar('Q', bound='Base[T]')
-
        def __init__(self, item: T) -> None:
            self.item = item