atomicwrites: add pyi file (#705)

Authorized by atomicwrites author here:
https://github.com/untitaker/python-atomicwrites/pull/23#issuecomment-260422021

third_party/2and3/atomicwrites/__init__.pyi

@@ -0,0 +1,16 @@
+import contextlib
+import os
+import sys
+import tempfile
+from typing import Any, AnyStr, Callable, IO, Iterator, Text
+def replace_atomic(src: AnyStr, dst: AnyStr) -> None: ...
+def move_atomic(src: AnyStr, dst: AnyStr) -> None: ...
+class AtomicWriter(object):
+    def __init__(self, path: AnyStr, mode: Text='w', overwrite: bool=False) -> None: ...
+    def open(self) -> contextlib.ContextManager[IO]: ...
+    def _open(self, get_fileobject: Callable) -> contextlib.ContextManager[IO]: ...
+    def get_fileobject(self, dir: AnyStr=None, **kwargs) -> IO: ...
+    def sync(self, f: IO) -> None: ...
+    def commit(self, f: IO) -> None: ...
+    def rollback(self, f: IO) -> None: ...
+def atomic_write(path: AnyStr, writer_cls: type=AtomicWriter, **cls_kwargs) -> contextlib.ContextManager[IO]: ...