Fix test for old Ubuntu

pystencils_tests/test_astnodes.py

+import pytest
 import sympy as sp
-import pystencils as ps
 
+import pystencils as ps
 from pystencils import Assignment
-from pystencils.astnodes import Block, SkipIteration, LoopOverCoordinate, SympyAssignment
-from sympy.codegen.rewriting import optims_c99
+from pystencils.astnodes import Block, LoopOverCoordinate, SkipIteration, SympyAssignment
 
 dst = ps.fields('dst(8): double[2D]')
 s = sp.symbols('s_:8')
@@ -83,7 +83,8 @@ def test_loop_over_coordinate():
 
 
 def test_sympy_assignment():
-
+    pytest.importorskip('sympy.codegen.rewriting')
+    from sympy.codegen.rewriting import optims_c99
     assignment = SympyAssignment(dst[0, 0](0), sp.log(x + 3) / sp.log(2) + sp.log(x ** 2 + 1))
     assignment.optimize(optims_c99)