Override os._Environ.__ior__ (#6921)

Supporting the `|=` operator for `os.environ` was introduced in Python 3.9. However,
this led to false negatives in type checking statements like `os.environ |= {"k": "v"}`
and `os.environ |= [("k", "v")]` because the definition inherited from
`typing.MutableMapping` was insufficient. This addresses that deficiency.

Fixes #6919.

stdlib/os/__init__.pyi

@@ -245,6 +245,13 @@ class _Environ(MutableMapping[AnyStr, AnyStr], Generic[AnyStr]):
     def __setitem__(self, key: AnyStr, value: AnyStr) -> None: ...
     def __iter__(self) -> Iterator[AnyStr]: ...
     def __len__(self) -> int: ...
+    if sys.version_info >= (3, 9):
+        # We use @overload instead of a Union for reasons similar to those given for
+        # overloading MutableMapping.update in stdlib/typing.pyi
+        @overload
+        def __ior__(self: Self, value: Mapping[AnyStr, AnyStr]) -> Self: ...
+        @overload
+        def __ior__(self: Self, value: Iterable[tuple[AnyStr, AnyStr]]) -> Self: ...
 
 environ: _Environ[str]
 if sys.platform != "win32":