py3.12: datetime.datetime.utcfromtimestamp() is deprecated and scheduled for removal

[graingert]

traceback:

HTTPServerRequest(protocol='https', host='localhost:46611', method='GET', uri='/statics/individual-cluster-map.html', version='HTTP/1.1', remote_ip='127.0.0.1')
Traceback (most recent call last):
  File "/home/graingert/mambaforge/envs/dask-distributed-312/lib/python3.12/site-packages/tornado/web.py", line 1786, in _execute
    result = await result
             ^^^^^^^^^^^^
  File "/home/graingert/mambaforge/envs/dask-distributed-312/lib/python3.12/site-packages/tornado/web.py", line 2688, in get
    self.modified = self.get_modified_time()
                    ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/graingert/mambaforge/envs/dask-distributed-312/lib/python3.12/site-packages/tornado/web.py", line 2994, in get_modified_time
    modified = datetime.datetime.utcfromtimestamp(int(stat_result.st_mtime))
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
DeprecationWarning: datetime.datetime.utcfromtimestamp() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.fromtimestamp(timestamp, datetime.UTC).

[graingert]

it looks like some of these were fixed in #3288

[bdarnell]

Yes, this will be fixed with tornado 6.4 as soon as I can figure out what's going on with some windows CI failures.

[graingert]

ah I see the one in the traceback was fixed in #3288 but a grep found some that aren't covered by the test suite

[bdarnell]

Fixed these in #3341 (and added a new test)