misc: replaced all "dummy" by the more conventional and readable "__" ("_" being used for gettext)

sat/plugins/plugin_xep_0363.py

@@ -148,7 +148,7 @@ class XEP_0363(object):
         profile=C.PROF_KEY_NONE,
     ):
         assert os.path.isabs(filepath) and os.path.isfile(filepath)
-        progress_id_d, dummy = self.fileHTTPUpload(
+        progress_id_d, __ = self.fileHTTPUpload(
             filepath,
             filename or None,
             jid.JID(upload_jid) if upload_jid else None,
@@ -239,7 +239,7 @@ class XEP_0363(object):
         )
         return d
 
-    def _uploadCb(self, dummy, sat_file, slot, download_d):
+    def _uploadCb(self, __, sat_file, slot, download_d):
         """Called once file is successfully uploaded
 
         @param sat_file(SatFile): file used for the upload