Calling an `ABIReturnSubroutine` instance returns the type `abi.ReturnedValue | Expr`

## Problem

When calling a subroutine decorated with `ABIReturnSubroutine`, the Python type annotation of the returned value is `abi.ReturnedValue | Expr`. Take a look at the definition of `ABIReturnSubroutine.__call__`: https://github.com/algorand/pyteal/blob/4c716b44819b9d529df98776423e74dac31883b1/pyteal/ast/subroutine.py#L663-L665

The rational behind this behavior is that if an `ABIReturnSubroutine` returns a value, calling it will return an `abi.ReturnedValue`; but if the subroutine does not return a valid (has void return type), then it returns a simple `Expr`.

This union type is problematic for calling code, since `abi.ReturnedValue` and `Expr` behave completely differently. As of right now, calling must must cast the value to one of `abi.ReturnedValue` or `Expr` for it to pass type checking.

## Solution

At the moment I can't think of a good solution for this. `ABIReturnSubroutine` instances really do return either `abi.ReturnedValue` or `Expr`, and I'm not aware of any tricks we can use in the Python type system to make it aware of this.