This is code I wrote. Python2 was able to determine what symbols were 
referenced in a zope python expression. Python3 needed a more complex 
approach. For python3 using the python2 approach didn't determine all of 
the symbols in a zope python expression. It broke one of our trackers and 
had to be fixed. Not sure why pypy3 has trouble with 'import symtable'.

If someone else has a way for python3 to determine which symbols are 
referenced in a zope python expression that should be examined.