Change requirements for pystencils_autodiff

setup.cfg

@@ -32,7 +32,8 @@ setup_requires = pyscaffold>=3.2a0,<3.3a0
 # Add here dependencies of your project (semicolon/line-separated), e.g.
 install_requires =
     numpy
-    pystencils>=0.2.7
+    pystencils>=0.2.8
+    sympy>=0.5 # lower sympy version will have problems with a bug in floor when using interpolation
     jinja2
     tqdm
     p_tqdm # for parallel compilation in tensorflow_jit