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

@@ -81,7 +81,10 @@ class OpenClSympyPrinter(CudaSympyPrinter):
     # For math functions, OpenCL is more similar to the C++ printer CustomSympyPrinter
     # since built-in math functions are generic.
     # In CUDA, you have to differentiate between `sin` and `sinf`
-    _print_math_func = CustomSympyPrinter._print_math_func
+    try:
+        _print_math_func = CustomSympyPrinter._print_math_func
+    except AttributeError:
+        pass
     _print_Pow = CustomSympyPrinter._print_Pow
 
     def _print_Function(self, expr):