tests/basics: Add test for dict.fromkeys where arg is a generator.

Improves coverage because it tests the case where the arg does not have a
__len__ slot.

tests/basics/dict_fromkeys.py

@@ -8,3 +8,6 @@ l = list(d.values())
 l.sort()
 print(l)
 
+# argument to fromkeys is a generator
+d = dict.fromkeys(i + 1 for i in range(1))
+print(d)