Add tarfile.filemode. (#2869)

This function is defined up to Python 3.7 (albeit deprecated in Python 3).

stdlib/2and3/tarfile.pyi

@@ -8,7 +8,6 @@ import os
 import sys
 from types import TracebackType
 
-
 if sys.version_info >= (3, 6):
     _Path = Union[bytes, str, os.PathLike]
 elif sys.version_info >= (3,):
@@ -49,7 +48,6 @@ def open(name: Optional[_Path] = ..., mode: str = ...,
          errorlevel: Optional[int] = ...,
          compresslevel: Optional[int] = ...) -> TarFile: ...
 
-
 class TarFile(Iterable[TarInfo]):
     name = ...  # type: Optional[_Path]
     mode = ...  # type: str
@@ -140,16 +138,16 @@ class TarFile(Iterable[TarInfo]):
                    fileobj: Optional[IO[bytes]] = ...) -> TarInfo: ...
     def close(self) -> None: ...
 
-
 def is_tarfile(name: str) -> bool: ...
 
+if sys.version_info < (3, 8):
+    def filemode(mode: int) -> str: ...  # undocumented
 
 if sys.version_info < (3,):
     class TarFileCompat:
         def __init__(self, filename: str, mode: str = ...,
                      compression: int = ...) -> None: ...
 
-
 class TarError(Exception): ...
 class ReadError(TarError): ...
 class CompressionError(TarError): ...
@@ -157,7 +155,6 @@ class StreamError(TarError): ...
 class ExtractError(TarError): ...
 class HeaderError(TarError): ...
 
-
 class TarInfo:
     name = ...  # type: str
     size = ...  # type: int