Merge branch 'c11-printer' into 'master'

Use C11CodePrinter for sympy 1.7

See merge request !176

pystencils/backends/cbackend.py

@@ -23,7 +23,7 @@ except ImportError:
     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
+        from sympy.printing.c import C11CodePrinter as CCodePrinter  # for sympy versions > 1.6
 
 __all__ = ['generate_c', 'CustomCodeNode', 'PrintNode', 'get_headers', 'CustomSympyPrinter']