Support both python3-progressbar and python3-progressbar2, two modules...

Support both python3-progressbar and python3-progressbar2, two modules providing the "progressbar" Python module. (Closes: #323)

diffoscope/progress.py

@@ -267,7 +267,7 @@ class ProgressBar:
         self.msg = msg
 
         self.bar.maxval = total
-        self.bar.currval = current
+        self.bar.value = current
         self.bar.update()
 
     def finish(self):