fix VOIP crashes under Python 3

changelog.d/3835.bugfix

@@ -0,0 +1 @@
+fix VOIP crashes under Python 3 (#3821)

synapse/rest/client/v1/voip.py

@@ -42,7 +42,11 @@ def on_GET(self, request):
             expiry = (self.hs.get_clock().time_msec() + userLifetime) / 1000
             username = "%d:%s" % (expiry, requester.user.to_string())
 
-            mac = hmac.new(turnSecret, msg=username, digestmod=hashlib.sha1)
+            mac = hmac.new(
+                turnSecret.encode(),
+                msg=username.encode(),
+                digestmod=hashlib.sha1
+            )
             # We need to use standard padded base64 encoding here
             # encode_base64 because we need to add the standard padding to get the
             # same result as the TURN server.