Binder doesn't understand `if MYPY:` branches are taken unconditionally

For example, this code leaves `x` with type `A` on the last line, when it should have type `B`:
```python
class A: pass
class B(A): pass

MYPY = False

def f() -> None:
    x = A()
    if MYPY: assert isinstance(x, B)
    reveal_type(x)
```