[PyROOT] Py3: Clear Python error before getting attribute

In Python3, if an error is set (e.g. because we tried to get an
attribute that does not exist from an object), any subsequent
get attribute operation will fail, even if the attribute is
present. Clearing the error after the first (failed) attribute
get solves the problem.

bindings/pyroot/src/TemplateProxy.cxx

@@ -276,7 +276,10 @@ namespace {
    // still here? try instantiating methods
 
       PyObject* clName = PyObject_GetAttr( pytmpl->fPyClass, PyStrings::gCppName );
-      if ( ! clName ) clName = PyObject_GetAttr( pytmpl->fPyClass, PyStrings::gName );
+      if (!clName) {
+         PyErr_Clear();
+         clName = PyObject_GetAttr(pytmpl->fPyClass, PyStrings::gName);
+      }
       auto clNameStr = std::string(PyROOT_PyUnicode_AsString(clName));
       if (clNameStr == "_global_cpp")
          clNameStr = ""; // global namespace