Fix deprecation warning for Sympy 1.7

We have to try from newest to oldest import to avoid deprecation warnings.

They added the old import again, but with a deprecation warning.

pystencils/backends/cbackend.py

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