fix depecrated import of python.pxi

Author: rwst

src/sage/symbolic/comparison.pyx

@@ -12,8 +12,7 @@ There are two useful ways to compare symbolic expressions:
   symbolic variables). Can be very slow as it potentially calls
   Maxima to prove the inequality.
 """
-
-include "sage/ext/python.pxi"
+from cpython cimport *
 
 from sage.symbolic.ring import SR
 from sage.symbolic.expression cimport is_Expression