fix list.copy to use the expected type

Author: KotlinIsland

stdlib/@tests/test_cases/builtins/check_list.py

@@ -19,3 +19,8 @@ def asd(self) -> int:
 assert_type(combined, List[Union[Foo, Bar]])
 for item in combined:
     assert_type(item.asd(), int)
+
+l1: list[int] = [1]
+l2: list[object] = l1.copy()
+# this is an error, because a list of ints can't be a list of strs
+l3: list[str] = l1.copy()  # type: ignore

stdlib/builtins.pyi

@@ -1104,7 +1104,8 @@ class list(MutableSequence[_T]):
     def __init__(self) -> None: ...
     @overload
     def __init__(self, iterable: Iterable[_T], /) -> None: ...
-    def copy(self) -> list[_T]: ...
+    # `copy` returns a new object, so capture the expected return type here using a type var
+    def copy(self) -> list[_S | _T]: ...
     def append(self, object: _T, /) -> None: ...
     def extend(self, iterable: Iterable[_T], /) -> None: ...
     def pop(self, index: SupportsIndex = -1, /) -> _T: ...