Support named attributes in os.uname() result (#1445)

`os.uname` changed in version 3.3: Return type changed from a tuple to
a tuple-like object with named attributes.

stdlib/3/posix.pyi

@@ -2,5 +2,11 @@
 
 # NOTE: These are incomplete!
 
+import sys
 import typing
 from os import stat_result
+from typing import NamedTuple
+
+if sys.version_info >= (3, 3):
+    uname_result = NamedTuple('uname_result', [('sysname', str), ('nodename', str),
+        ('release', str), ('version', str), ('machine', str)])