Deprecate treating naive datetimes as UTC

[RazerM]

LoggingHandler and RedirectLoggingHandler need to be able to convert LogRecord.time to a timestamp, but logbook.set_datetime_format means it is impossible to know what a naive datetime means.

Logbook should match the Python standard library and treat naive datetimes as local time.

Plan

This will be a breaking change, i.e. Logbook 2.0

• Remove logbook.set_datetime_format.
• Change LogRecord.time to always use aware datetimes. May wish to have Logger pass down a timezone which defaults to timezone.utc.

• Add a tzinfo attribute to Handler which does record.time.astimezone(tzinfo) if it is set to None or datetime.tzinfo. This allows having handlers convert to different timezones, similar to logging.Formatter.convert. We'll need to make this work in the format string, since currently you do record.time.

  Converting to a timezone is similar performance to the conversion logging.Formatter already does:

  In [1]: from time import localtime, gmtime
  
  In [2]: from datetime import datetime, timezone
  
  In [3]: from zoneinfo import ZoneInfo
  
  In [4]: now = datetime.now(timezone.utc)
  
  In [5]: nowts = now.timestamp()
  
  In [6]: berlin = ZoneInfo('Europe/Berlin')
  
  In [7]: %timeit localtime(nowts)
  331 ns ± 0.701 ns per loop (mean ± std. dev. of 7 runs, 1,000,000 loops each)
  
  In [8]: %timeit gmtime(nowts)
  218 ns ± 0.35 ns per loop (mean ± std. dev. of 7 runs, 1,000,000 loops each)
  
  In [9]: %timeit now.astimezone(berlin)
  195 ns ± 0.657 ns per loop (mean ± std. dev. of 7 runs, 1,000,000 loops each)
  
  In [10]: %timeit now.astimezone(timezone.utc)
  45 ns ± 0.104 ns per loop (mean ± std. dev. of 7 runs, 10,000,000 loops each)