Make PyAlg::_obj public to avoid visibility issues in Python

see https://sft.its.cern.ch/jira/browse/ROOT-10872?focusedCommentId=109891&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-109891

GaudiPython/GaudiPython/Algorithm.h

@@ -81,9 +81,6 @@ namespace GaudiPython {
       this->addRef();
       this->addRef();
     }
-    // ========================================================================
-  protected:
-    // ========================================================================
     /// get the object
     PyObject* _obj() const { return m_self; } //     get the object
     // ========================================================================