Fix python 2 string iterators.

test/completion/comprehensions.py

@@ -89,6 +89,9 @@ b = [b for arr in [[[1.0]]] for a in arr for b in a]
 #? float()
 b[0]
 
+#? str()
+[x for x in 'chr'][0]
+
 # jedi issue #26
 #? list()
 a = [[int(v) for v in line.strip().split() if v] for line in ["123", "123", "123"] if line]