Switch core module to Python3

The core module (the relational algebra, not the interface) is now
using Python3

Tests are passing, so it should be ok.

test/rename_insert2.exec

@@ -1,7 +1,5 @@
 p1=people.rename({"id":"ido"})
 p1.insert((123,"lala",0,31))
 
-print p1
-print people
 retval=people==p1
 people.delete("id==123")