[Cython] Fix treatment of writelogendl()

Both writelog and writelogendl should be passed through PySys_WriteStdout to
ensure that they are printed in the correct order. Problems with this were
identified in the 1D solver log when using Python 3.

include/cantera/cython/wrappers.h

@@ -27,6 +27,10 @@ public:
         }
     }
 
+    virtual void writeendl() {
+        PySys_WriteStdout("%s", "\n");
+    }
+
     virtual void error(const std::string& msg) {
         std::string err = "raise Exception('''"+msg+"''')";
         PyRun_SimpleString(err.c_str());