merge 2 and 3 stubs for fileinput (#1319)

* merge 2 and 3 stubs for fileinput

Plus some minor fixes.

* pytype doesn't like generic type aliases

stdlib/3/fileinput.pyi

@@ -1,57 +0,0 @@
-from typing import Iterable, Callable, IO, AnyStr, Generic, Any, Union, Iterator, Optional
-
-import os
-import sys
-
-if sys.version_info >= (3, 6):
-    _Path = Union[str, bytes, os.PathLike[Any]]
-else:
-    _Path = Union[str, bytes]
-_Opener = Callable[[_Path, str], IO[AnyStr]]
-
-
-def input(
-    files: Union[str, Iterable[str]]=None,
-    inplace: bool=...,
-    backup: str=...,
-    bufsize: int=...,
-    mode: str=...,
-    openhook: Callable[[str, str], IO[AnyStr]]=...) -> Iterable[AnyStr]: ...
-
-
-def close() -> None: ...
-def nextfile() -> None: ...
-def filename() -> str: ...
-def lineno() -> int: ...
-def isfirstline() -> bool: ...
-def isstdin() -> bool: ...
-
-class FileInput(Iterable[AnyStr], Generic[AnyStr]):
-    def __init__(
-        self,
-        files: Union[None, _Path, Iterable[_Path]] = ...,
-        inplace: bool = ...,
-        backup: str = ...,
-        bufsize: int = ...,
-        mode: str = ...,
-        openhook: _Opener[AnyStr] = ...
-    ) -> None: ...
-
-    def __del__(self) -> None: ...
-    def close(self) -> None: ...
-    def __enter__(self) -> 'FileInput[AnyStr]': ...
-    def __exit__(self, type: Any, value: Any, traceback: Any) -> None: ...
-    def __iter__(self) -> Iterator[AnyStr]: ...
-    def __next__(self) -> AnyStr: ...
-    def __getitem__(self, i: int) -> AnyStr: ...
-    def nextfile(self) -> None: ...
-    def readline(self) -> AnyStr: ...
-    def filename(self) -> str: ...
-    def lineno(self) -> int: ...
-    def filelineno(self) -> int: ...
-    def fileno(self) -> int: ...
-    def isfirstline(self) -> bool: ...
-    def isstdin(self) -> bool: ...
-
-def hook_compressed(filename: _Path, mode: str) -> IO[Any]: ...
-def hook_encoded(encoding: str, errors: Optional[str] = ...) -> _Opener[Any]: ...