Add stub for statvfs and type for fstatvfs (2.7) (#311)

Use the same approach as for resource: define a private NamedTuple
to represent the return value that can be addressed by name or by
sequence.

stdlib/2.7/os/__init__.pyi

@@ -1,7 +1,8 @@
 # created from https://docs.python.org/2/library/os.html
 
 from typing import (
-    List, Tuple, Union, Sequence, Mapping, IO, Any, Optional, AnyStr, Iterator, MutableMapping
+    List, Tuple, Union, Sequence, Mapping, IO, Any, Optional, AnyStr, Iterator,
+    MutableMapping, NamedTuple
 )
 import os.path as path
 
@@ -76,7 +77,6 @@ def fpathconf(fd: int, name: str) -> None: ...
 
 # TODO(prvak)
 def fstat(fd: int) -> Any: ...
-def fstatvfs(fd: int) -> Any: ...
 def fsync(fd: int) -> None: ...
 def ftruncate(fd: int, length: int) -> None: ...
 def isatty(fd: int) -> bool: ...
@@ -140,7 +140,15 @@ def rmdir(path: unicode) -> None: ...
 # TODO(MichalPokorny)
 def stat(path: unicode) -> Any: ...
 
-# TODO: stat_float_times, statvfs, tempnam, tmpnam, TMP_MAX
+_StatVFS = NamedTuple('_StatVFS', [('f_bsize', int), ('f_frsize', int), ('f_blocks', int),
+                                   ('f_bfree', int), ('f_bavail', int), ('f_files', int),
+                                   ('f_ffree', int), ('f_favail', int), ('f_flag', int),
+                                   ('f_namemax', int)])
+
+def fstatvfs(fd: int) -> _StatVFS: ...
+def statvfs(path: unicode) -> _StatVFS: ...
+
+# TODO: stat_float_times, tempnam, tmpnam, TMP_MAX
 def walk(top: AnyStr, topdown: bool = ..., onerror: Any = ...,
          followlinks: bool = ...) -> Iterator[Tuple[AnyStr, List[AnyStr],
                                                     List[AnyStr]]]: ...