Improve some in-place BinOp methods (#7149)

stubs/setuptools/pkg_resources/__init__.pyi

@@ -1,6 +1,7 @@
 import importlib.abc
 import types
 import zipimport
+from _typeshed import Self
 from abc import ABCMeta
 from typing import IO, Any, Callable, Generator, Iterable, Optional, Sequence, TypeVar, Union, overload
 
@@ -53,7 +54,7 @@ class Environment:
     def remove(self, dist: Distribution) -> None: ...
     def can_add(self, dist: Distribution) -> bool: ...
     def __add__(self, other: Distribution | Environment) -> Environment: ...
-    def __iadd__(self, other: Distribution | Environment) -> Environment: ...
+    def __iadd__(self: Self, other: Distribution | Environment) -> Self: ...
     @overload
     def best_match(self, req: Requirement, working_set: WorkingSet) -> Distribution: ...
     @overload