Make io._IOBase and its descendents concrete. (#1172)

This solves the issue with instantiating gzip.GzipFile() mentioned at
https://github.com/python/typeshed/pull/1160#issuecomment-294534004

stdlib/2/_io.pyi

@@ -1,4 +1,4 @@
-from typing import Any, BinaryIO, Optional, Iterable, Tuple, List, Union
+from typing import Any, BinaryIO, IO, Iterable, Iterator, List, Optional, Type, Tuple, Union
 
 DEFAULT_BUFFER_SIZE = ...  # type: int
 
@@ -14,6 +14,28 @@ class _IOBase(BinaryIO):
     def _checkReadable(self) -> None: ...
     def _checkSeekable(self) -> None: ...
     def _checkWritable(self) -> None: ...
+    # All these methods are concrete here (you can instantiate this)
+    def close(self) -> None: ...
+    def fileno(self) -> int: ...
+    def flush(self) -> None: ...
+    def isatty(self) -> bool: ...
+    def read(self, n: int = ...) -> bytes: ...
+    def readable(self) -> bool: ...
+    def readline(self, limit: int = ...) -> bytes: ...
+    def readlines(self, hint: int = ...) -> list[bytes]: ...
+    def seek(self, offset: int, whence: int = ...) -> int: ...
+    def seekable(self) -> bool: ...
+    def tell(self) -> int: ...
+    def truncate(self, size: Optional[int] = ...) -> int: ...
+    def writable(self) -> bool: ...
+    def write(self, s: bytes) -> int: ...
+    def writelines(self, lines: Iterable[bytes]) -> None: ...
+    def next(self) -> bytes: ...
+    def __iter__(self) -> Iterator[bytes]: ...
+    def __enter__(self) -> '_IOBase': ...
+    def __exit__(self, t: Optional[Type[BaseException]], value: Optional[BaseException],
+                 # TODO: traceback should be TracebackType but that's defined in types
+                 traceback: Optional[Any]) -> bool: ...
 
 class _BufferedIOBase(_IOBase):
     def read1(self, n: int) -> str: ...

stdlib/2/io.pyi

@@ -101,4 +101,4 @@ class TextIOWrapper(TextIO):
     def __enter__(self) -> StringIO: ...
     def __exit__(self, type, value, traceback) -> bool: ...
 
-class BufferedIOBase(IOBase): ...
+class BufferedIOBase(_io._BufferedIOBase, IOBase): ...