Fix type hinting for .joinpath

Type hinting was taking its cue from the multimethod decorator, so
joinpath was always returning Any as the type. To resolve, the
multimethod decorator has been annotated with generics so that the
typing system can correctly infer the return type of joinpath.

path/classes.pyi

@@ -1,8 +1,13 @@
-from typing import Any, Callable, Optional
+from typing import Any, Callable, Generic, TypeVar, overload
 
 class ClassProperty(property):
     def __get__(self, cls: Any, owner: type | None = ...) -> Any: ...
 
-class multimethod:
-    def __init__(self, func: Callable[..., Any]): ...
-    def __get__(self, instance: Any, owner: type | None) -> Any: ...
+_T = TypeVar("_T")
+
+class multimethod(Generic[_T]):
+    def __init__(self, func: Callable[..., _T]): ...
+    @overload
+    def __get__(self, instance: None, owner: type[_T] | None) -> Callable[..., _T]: ...
+    @overload
+    def __get__(self, instance: _T, owner: type[_T] | None) -> Callable[..., _T]: ...