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.py

@@ -1,5 +1,7 @@
+from __future__ import annotations
+
 import functools
-from typing import Any, Callable
+from typing import Any, Callable, Generic, TypeVar
 
 
 class ClassProperty(property):
@@ -8,18 +10,21 @@ def __get__(self, cls: Any, owner: type | None = None) -> Any:
         return self.fget.__get__(None, owner)()
 
 
-class multimethod:
+_T = TypeVar("_T")
+
+
+class multimethod(Generic[_T]):
     """
     Acts like a classmethod when invoked from the class and like an
     instancemethod when invoked from the instance.
     """
 
-    func: Callable[..., Any]
+    func: Callable[..., _T]
 
-    def __init__(self, func: Callable[..., Any]):
+    def __init__(self, func: Callable[..., _T]):
         self.func = func
 
-    def __get__(self, instance: Any | None, owner: type | None) -> Any:
+    def __get__(self, instance: _T | None, owner: type[_T] | None) -> Callable[..., _T]:
         """
         If called on an instance, pass the instance as the first
         argument.