tarfile.TarFile: mark OPEN_METH as a ClassVar (#8725)

stdlib/tarfile.pyi

@@ -6,7 +6,7 @@ from builtins import list as _list, type as Type  # aliases to avoid name clashe
 from collections.abc import Callable, Iterable, Iterator, Mapping
 from gzip import _ReadableFileobj as _GzipReadableFileobj, _WritableFileobj as _GzipWritableFileobj
 from types import TracebackType
-from typing import IO, Protocol, overload
+from typing import IO, ClassVar, Protocol, overload
 from typing_extensions import Literal
 
 __all__ = [
@@ -110,7 +110,7 @@ class ExFileObject(io.BufferedReader):
     def __init__(self, tarfile: TarFile, tarinfo: TarInfo) -> None: ...
 
 class TarFile:
-    OPEN_METH: Mapping[str, str]
+    OPEN_METH: ClassVar[Mapping[str, str]]
     name: StrOrBytesPath | None
     mode: Literal["r", "a", "w", "x"]
     fileobj: _Fileobj | None