Fix checksum following BIP-128 standard

Non-Ascii characters should not be converted for
checksum calculation.
This will give consistent hash to BIP-128 and its
Javascript code example.

Timelock-Recovery Plans that contained only ascii
characters are not affected.

Also, 8 hex-chars is enough for a checksum.

electrum/plugins/timelock_recovery/qt.py

@@ -687,10 +687,10 @@ class Plugin(TimelockRecoveryPlugin):
         # object whose fields can be ordered in multiple ways).
         return hashlib.sha256(json.dumps(
             sorted(json_data.items()),
-            skipkeys=False, ensure_ascii=True, check_circular=True,
+            skipkeys=False, ensure_ascii=False, check_circular=True,
             allow_nan=True, cls=None, indent=None, separators=(',', ':'),
             default=None, sort_keys=False,
-        ).encode()).hexdigest()
+        ).encode()).hexdigest()[:8]
 
     def _save_recovery_plan_json(self, context: TimelockRecoveryContext, download_dialog: WindowModalDialog):
         try: