adapt opytex import with pyMath new stucture

opytex/opytex.py

@@ -11,11 +11,11 @@ from .texenv import texenv
 
 import math as m
 import random as rd
-from pymath.expression import Expression
-from pymath.polynom import Polynom
-from pymath.polynomDeg2 import Polynom_deg2
-from pymath.fraction import Fraction 
-from pymath.random_expression import random_str 
+from pymath import Expression
+from pymath import Polynom
+from pymath.calculus.polynomDeg2 import Polynom_deg2
+from pymath import Fraction 
+from pymath import random_str 
 
 
 export_dict = {}