Fix OpenCL with older Sympy and pyopencl versions

I'm using sympy and pyopencl from the Ubuntu 18.04 repositories and these changes were required to make the OpenCL tests pass on an Nvidia machine. AMD doesn't work, see #15 (closed).

pystencils/backends/opencl_backend.py

@@ -83,7 +83,7 @@ class OpenClSympyPrinter(CudaSympyPrinter):
     # In CUDA, you have to differentiate between `sin` and `sinf`
     try:
         _print_math_func = CustomSympyPrinter._print_math_func
-    except:
+    except AttributeError:
         pass
     _print_Pow = CustomSympyPrinter._print_Pow