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

In Python, you can change the environment by writing to os.environ.

This seems to already be handled correctly in the python 3 type data,
so we just need to fix it for 2.7.

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: ...