Use SHA256 for Note checksum

>> Issue: [B303:blacklist] Use of insecure MD2, MD4, MD5, or SHA1 hash function.
   Severity: Medium   Confidence: High
   Location: nxdrive/objects.py:165
   More Info: https://bandit.readthedocs.io/en/latest/blacklists/blacklist_calls.html#b303-md5
164                     else:
165                         m = hashlib.md5()
166                         m.update(note.encode("utf-8"))