Use C99 also on sympy 1.7

pystencils/backends/cbackend.py

@@ -18,7 +18,7 @@ from pystencils.integer_functions import (
     int_div, int_power_of_2, modulo_ceil)
 
 try:
-    from sympy.printing.c import C11CodePrinter as CCodePrinter  # for sympy versions > 1.6
+    from sympy.printing.c import C99CodePrinter as CCodePrinter  # for sympy versions > 1.6
 except ImportError:
     try:
         from sympy.printing.ccode import C99CodePrinter as CCodePrinter