Python: Re-introduce syntactic handling of str/bytes/unicode

I don't want to loose results on this, so until type-tracking/API graphs
can handle this, I want to keep our syntactic handling.

python/ql/test/experimental/dataflow/tainttracking/defaultAdditionalTaintStep/test_string.py

@@ -32,7 +32,7 @@ def str_operations():
         ts[0], # $ tainted
         str(ts), # $ tainted
         bytes(tb), # $ tainted
-        unicode(ts), # $ MISSING: tainted
+        unicode(ts), # $ tainted
     )
 
     aug_assignment = "safe"