Python 3.12 deprecation warning for date function

[tomwaldnz]

This is a great program, thanks for writing and maintaining it! I was about to write something myself, but please to find an existing tested tool that does pretty much exactly what I want :)

When I run on Windows with python 3.12.0 I get the following deprecation warning. It's probably a fairly easy fix, and it's not at all urgent.

bitrot.py:73: DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.now(datetime.UTC).
  return datetime.datetime.utcnow().strftime('%Y-%m-%d %H:%M:%S%z')