Fix deprecation warning for Sympy 1.7

We have to try from newest to oldest import to avoid deprecation warnings.

They added the old import again, but with a deprecation warning.

pystencils_tests/test_astnodes.py

@@ -5,12 +5,20 @@ import pystencils as ps
 from pystencils import Assignment
 from pystencils.astnodes import Block, LoopOverCoordinate, SkipIteration, SympyAssignment
 
+sympy_numeric_version = [int(x, 10) for x in sp.__version__.split('.')]
+if len(sympy_numeric_version) < 3:
+    sympy_numeric_version.append(0)
+sympy_numeric_version.reverse()
+sympy_version = sum(x * (100 ** i) for i, x in enumerate(sympy_numeric_version))
+
 dst = ps.fields('dst(8): double[2D]')
 s = sp.symbols('s_:8')
 x = sp.symbols('x')
 y = sp.symbols('y')
 
 
+@pytest.mark.skipif(sympy_version < 10501,
+                    reason="Old Sympy Versions behave differently which wont be supported in the near future")
 def test_kernel_function():
     assignments = [
         Assignment(dst[0, 0](0), s[0]),
@@ -36,6 +44,8 @@ def test_skip_iteration():
     assert skipped.undefined_symbols == set()
 
 
+@pytest.mark.skipif(sympy_version < 10501,
+                    reason="Old Sympy Versions behave differently which wont be supported in the near future")
 def test_block():
     assignments = [
         Assignment(dst[0, 0](0), s[0]),