Python 3.12 deprecates the utcnow and utcfromtimestamp methods and
discourages the use of naive datetimes to represent UTC. This was
previously the main way that Tornado used datetimes (since it was
the only option available in Python 2 before the introduction
of datetime.timezone.utc in Python 3.2).

- httpclient_test: Test-only change to test that both kinds of datetimes
  are supported in If-Modified-Since (this just calls
  httputil.format_timestamp)
- httputil: No functional changes, but format_timestamp's
  support for both naive and aware datetimes is now tested.
- locale: format_timestamp now supports aware datetimes (in
  addition to the existing support for naive datetimes).
- web: Cookie expirations internally use aware datetimes.
  StaticFileHandler.get_modified_time now supports both and the
  standard implementation returns aware.

It feels fragile that "naive" and "aware" datetimes are not distinct
types but subject to data-dependent behavior. This change uses
"aware" datetimes throughout Tornado, but some operations (comparisons
and subtraction) fail with mixed datetime types and if I missed any
in this change may cause errors if naive datetimes were used (where
previously naive datetimes would have been required). But that's
apparently the API we have to work with.