builtins: mode argument to open can be unicode (#1452)

Part of #1411

Also remove some explicit parameter defaults.

Author: JelleZijlstra

stdlib/2/__builtin__.pyi

@@ -750,14 +750,14 @@ def next(i: Iterator[_T]) -> _T: ...
 def next(i: Iterator[_T], default: _VT) -> Union[_T, _VT]: ...
 def oct(i: int) -> str: ...  # TODO __index__
 @overload
-def open(file: str, mode: str = 'r', buffering: int = ...) -> BinaryIO: ...
+def open(file: str, mode: unicode = ..., buffering: int = ...) -> BinaryIO: ...
 @overload
-def open(file: unicode, mode: str = 'r', buffering: int = ...) -> BinaryIO: ...
+def open(file: unicode, mode: unicode = ..., buffering: int = ...) -> BinaryIO: ...
 @overload
-def open(file: int, mode: str = 'r', buffering: int = ...) -> BinaryIO: ...
+def open(file: int, mode: unicode = ..., buffering: int = ...) -> BinaryIO: ...
 def ord(c: unicode) -> int: ...
 # This is only available after from __future__ import print_function.
-def print(*values: Any, sep: unicode = u' ', end: unicode = u'\n',
+def print(*values: Any, sep: unicode = ..., end: unicode = ...,
            file: IO[Any] = ...) -> None: ...
 @overload
 def pow(x: int, y: int) -> Any: ...  # The return type can be int or float, depending on y.
@@ -768,7 +768,7 @@ def pow(x: float, y: float) -> float: ...
 @overload
 def pow(x: float, y: float, z: float) -> float: ...
 def quit(code: int = ...) -> None: ...
-def range(x: int, y: int = 0, step: int = 1) -> List[int]: ...
+def range(x: int, y: int = ..., step: int = ...) -> List[int]: ...
 def raw_input(prompt: Any = ...) -> str: ...
 
 @overload