py: Simplify and remove redundant code for __iter__ method lookup.

tests/basics/iter0.py

@@ -0,0 +1,6 @@
+# builtin type that is not iterable
+try:
+    for i in 1:
+        pass
+except TypeError:
+    print('TypeError')

tests/basics/iter1.py

@@ -1,5 +1,14 @@
 # test user defined iterators
 
+# this class is not iterable
+class NotIterable:
+    pass
+try:
+    for i in NotIterable():
+        pass
+except TypeError:
+    print('TypeError')
+
 class MyStopIteration(StopIteration):
     pass