Merge branch 'fix-compilation-with-python-3' into 'master'

Fix compilation with Python 3

See merge request !44

dune/fufem/python/reference.hh

@@ -335,7 +335,7 @@ class Reference
         {
             assertPyObject("str()");
 #if PY_MAJOR_VERSION >= 3
-            char* s = PyUnicode_AsUTF8(PyObject_Str(p_));
+            const char* s = PyUnicode_AsUTF8(PyObject_Str(p_));
 #else
             char* s = PyString_AsString(PyObject_Str(p_));
 #endif