Convert qubes-input-trigger to python3

Fedora 30 require explicit python version in shebang. Since the script
is very simple, use this occasion to make it python3 compatible.

QubesOS/qubes-issues#4845

qubes-rpc/qubes-input-trigger

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/python3
 #
 # Trigger "add" udev even for all input mouse/keyboard devices.
 # Used to retry qrexec confirmation dialogs after user login
@@ -14,7 +14,7 @@ for input_dev in eventFiles:
         eventFile = os.path.join("/dev/input", input_dev)
         if S_ISCHR(os.stat(eventFile).st_mode) != 0:  # is character device?
             udevreturn = subprocess.check_output([
-                "udevadm", "info", "--query=property", "--name=" + eventFile])
+                "udevadm", "info", "--query=property", "--name=" + eventFile]).decode()
             if 'ID_INPUT_MOUSE' in udevreturn or \
                     'ID_INPUT_KEYBOARD' in udevreturn or \
                     'ID_INPUT_TABLET' in udevreturn or \