py/objgenerator: Implement PEP479, StopIteration convs to RuntimeError.

This commit implements PEP479 which disallows raising StopIteration inside
a generator to signal that it should be finished.  Instead, the generator
should simply return when it is complete.

See https://www.python.org/dev/peps/pep-0479/ for details.

tests/basics/generator_close.py

@@ -31,13 +31,14 @@ except StopIteration:
     print("StopIteration")
 
 
-# Throwing StopIteration in response to close() is ok
+# Throwing GeneratorExit in response to close() is ok
 def gen2():
     try:
         yield 1
         yield 2
     except:
-        raise StopIteration
+        print('raising GeneratorExit')
+        raise GeneratorExit
 
 g = gen2()
 next(g)