Fix typing of Animation (#3568)

Author: dandavison

manim/animation/animation.py

@@ -18,6 +18,8 @@
 from copy import deepcopy
 from typing import TYPE_CHECKING, Callable, Iterable, Sequence
 
+from typing_extensions import Self
+
 if TYPE_CHECKING:
     from manim.scene.scene import Scene
 
@@ -112,7 +114,7 @@ def __new__(
         *args,
         use_override=True,
         **kwargs,
-    ):
+    ) -> Self:
         if isinstance(mobject, Mobject) and use_override:
             func = mobject.animation_override_for(cls)
             if func is not None:

manim/typing.py

@@ -561,7 +561,9 @@
 # Due to current limitations
 # (see https://github.com/python/mypy/issues/14656 / 8263),
 # we don't specify the first argument type (Mobject).
-FunctionOverride: TypeAlias = Callable[..., None]
+# Nor are we able to specify the return type (Animation) since we cannot import
+# that here.
+FunctionOverride: TypeAlias = Callable
 """Function type returning an :class:`~.Animation` for the specified
 :class:`~.Mobject`.
 """