allow generator to return non None values

we create quite strange lambda cfg nodes but it should be fine I guess

src/runtime/generator.cpp

@@ -107,7 +107,6 @@ void generatorEntry(BoxedGenerator* g) {
             auto r = callCLFunc<ExceptionStyle::CXX, NOT_REWRITABLE>(func->md, nullptr, func->md->numReceivedArgs(),
                                                                      func->closure, g, func->globals, g->arg1, g->arg2,
                                                                      g->arg3, args);
-            assert(r == None);
             Py_DECREF(r);
         } catch (ExcInfo e) {
             // unhandled exception: propagate the exception to the caller

test/tests/generators.py

-# expected: reffail
 def G1(i=0):
     while True:
         yield i