Merge pull request #53 from timabbott/mutable

Fix os.environ improperly classified as not mutable for python 2.

stdlib/2.7/__builtin__.pyi

@@ -824,3 +824,4 @@ def cmp(x: Any, y: Any) -> int: ...
 def execfile(filename: str, globals: Dict[str, Any] = None, locals: Dict[str, Any] = None) -> None: ...
 
 class file(BinaryIO): ...
+

stdlib/2.7/os/__init__.pyi

@@ -1,11 +1,11 @@
 # created from https://docs.python.org/2/library/os.html
 
-from typing import List, Tuple, Union, Sequence, Mapping, IO, Any, Optional, AnyStr
+from typing import List, Tuple, Union, Sequence, Mapping, IO, Any, Optional, AnyStr, MutableMapping
 import os.path as path
 
 error = OSError
 name = ... # type: str
-environ = ... # type: Mapping[str, str]
+environ = ... # type: MutableMapping[str, str]
 
 def chdir(path: unicode) -> None: ...
 def fchdir(fd: int) -> None: ...