Make work on SymPy 1.7: sympy.printing.ccode -> sympy.printing.cxx

There's also sympy.printing.c but we are always compiling as C++.

sympy-master failed before this fix.

pystencils/backends/cbackend.py

@@ -20,7 +20,10 @@ from pystencils.integer_functions import (
 try:
     from sympy.printing.ccode import C99CodePrinter as CCodePrinter
 except ImportError:
-    from sympy.printing.ccode import CCodePrinter  # for sympy versions < 1.1
+    try:
+        from sympy.printing.ccode import CCodePrinter  # for sympy versions < 1.1
+    except ImportError:
+        from sympy.printing.cxx import CXX11CodePrinter as CCodePrinter  # for sympy versions > 1.6
 
 __all__ = ['generate_c', 'CustomCodeNode', 'PrintNode', 'get_headers', 'CustomSympyPrinter']