removing use of has_key() for Python 3 compatibility

ligo/gracedb/rest.py

@@ -1570,7 +1570,7 @@ class GraceDb(GsiRest):
             # Get actual response JSON
             r_json = response.json()
 
-            if r_json.has_key('voevents'):
+            if 'voevents' in r_json:
                 # list of voevents - iterate and get file text
                 for v in r_json['voevents']:
                     v[u'text'] = self.files(object_id, v['filename']).read()