Proof-of-concept for returning ModTime in detail responses

[duncanmmacleod]

This PR presents a proof-of-concept solution for #105.

Using the same example as in #105 the response is now:

In [2]: pelfs.ls("/igwn/ligo/frames/O4/", detail=True)
Out[2]:
[{'name': '/igwn/ligo/frames/O4/hoft_C00_AR/',
  'size': '4',
  'type': 'directory',
  'mod_time': datetime.datetime(2023, 8, 15, 21, 13, 16)},
 {'name': '/igwn/ligo/frames/O4/hoft_C00/',
  'size': '4',
  'type': 'directory',
  'mod_time': datetime.datetime(2023, 4, 25, 22, 27, 17)}]

[duncanmmacleod]

I have converted this to Draft because I'm working on #107, which should be a much better solution.