I think this is related to doc tests being executed in a namespace where the class definition is not available. I am not sure what is the best way here, a workaround is to explicitly pass the namespace, for example this passes:

import typing
  
def f(clazz, ns):
    """
    >>> class MyClass:
    ...   my_field: 'MyClass'
    >>> f(MyClass, globals())
    """
    typing.get_type_hints(clazz, ns)

You can maybe make the ns parameter optional, but it still doesn't look like a good solution.