plum: Does not work with future annotations

A resolution error is raised when you make a promise using annotations. It works if you stop importing the annotations package and use quotation marks.

The following code breaks. If you comment out the first line, the code works.

from __future__ import annotations
from plum import dispatch


class Displacement:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    @dispatch
    def __add__(self, other: "Displacement") -> "Displacement":
        return Displacement(self.x + other.x, self.y + other.y)

    @dispatch
    def __add__(self, other: int) -> "Displacement":
        return Displacement(self.x + other, self.y + other)


if __name__ == "__main__":
    d1 = Displacement(1, 2)
    d2 = Displacement(3, 4)
    print(d1 + d2)    

Traceback (most recent call last):
  File "C:\Users\gaspa\dev\python\py-chess\src\geometry_copy.py", line 22, in <module>
    print(d1 + d2)
  File "plum\function.py", line 585, in plum.function._BoundFunction.__call__
  File "plum\function.py", line 509, in plum.function.Function.__call__
  File "plum\function.py", line 373, in plum.function.Function._resolve_pending_registrations
  File "C:\Users\gaspa\.virtualenvs\py-chess-YZvcAOQM\lib\site-packages\plum\type.py", line 436, in is_object
    return t.get_types() == (object,)
  File "C:\Users\gaspa\.virtualenvs\py-chess-YZvcAOQM\lib\site-packages\plum\type.py", line 259, in get_types
    return ptype(self.resolve()).get_types()
  File "C:\Users\gaspa\.virtualenvs\py-chess-YZvcAOQM\lib\site-packages\plum\resolvable.py", line 41, in resolve
    raise ResolutionError(f"Promise `{self!r}` was not kept.")
plum.resolvable.ResolutionError: Promise `ForwardReferencedType(name="'Displacement'")` was not kept.

Process finished with exit code 1