don't try to use mpv on Windows

some users had mpv lying around from previous installs, and the Windows
implementation seems to hang with the idle hook introduced in the
previous build

aqt/main.py

@@ -308,6 +308,8 @@ close the profile or restart Anki."""))
     ##########################################################################
 
     def setupSound(self):
+        if isWin:
+            return
         try:
             anki.sound.setupMPV()
         except FileNotFoundError: