Revert "Make all single-constraint TypeVars to use bounds" (#806)

Reverts python/typeshed#804.

Reason: until python/mypy#1551 is fixed this gives an error whenever @skip() is used.

Specifically see https://github.com/python/typeshed/pull/804#issuecomment-269926655.

stdlib/2and3/plistlib.pyi

@@ -9,7 +9,7 @@ from enum import Enum
 import sys
 
 mm = MutableMapping[str, Any]
-_D = TypeVar('_D', bound=mm)
+_D = TypeVar('_D', mm)
 if sys.version_info >= (3,):
     _Path = str
 else: