[et_xmlfile] Complete the writer_cm annotation (#16075)

Author: eddiesr93

stubs/et_xmlfile/et_xmlfile/xmlfile.pyi

@@ -1,16 +1,15 @@
 import types
 import xml.etree.ElementTree as ET
-from _typeshed import Incomplete
-from collections.abc import Generator
-from contextlib import contextmanager
+from collections.abc import Callable, Generator
+from contextlib import _GeneratorContextManager, contextmanager
 from typing import Any
 
 class LxmlSyntaxError(Exception): ...
 
 class _IncrementalFileWriter:
     global_nsmap: dict[str, str]
     is_html: bool
-    def __init__(self, output_file: ET._FileWrite) -> None: ...
+    def __init__(self, output_file: Callable[[str], object]) -> None: ...
     @contextmanager
     def element(
         self,
@@ -27,7 +26,7 @@ class _IncrementalFileWriter:
 
 class xmlfile:
     encoding: str
-    writer_cm: Incomplete
+    writer_cm: _GeneratorContextManager[tuple[Callable[[str], object], str]] | None
     def __init__(
         self, output_file: ET._FileWrite, buffered: bool = False, encoding: str = "utf-8", close: bool = False
     ) -> None: ...