Update random.pyi

SystemRandom derives from Random (see http://svn.python.org/projects/python/branches/py3k/Lib/random.py).

stdlib/3/random.pyi

@@ -39,7 +39,7 @@ class Random(_random.Random):
     def weibullvariate(self, alpha: float, beta: float) -> float: ...
 
 # SystemRandom is not implemented for all OS's; good on Windows & Linux
-class SystemRandom:
+class SystemRandom(Random):
     def __init__(self, randseed: object = ...) -> None: ...
     def random(self) -> float: ...
     def getrandbits(self, k: int) -> int: ...