Make all single-constraint TypeVars use bound= insteads (plus hack) (#1118)

Another attempt for #804, to unblock python/mypy#2626.
There's a work-around here for python/mypy#1551.

stdlib/2and3/plistlib.pyi

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