Fix DateTimeError for non-valid timezones

[xispa]

Description of the issue/feature this PR addresses

This Pull Request fixes a DateTimeError when converting datetime with old/non-valid TZ to DateTime:

(Pdb) dt
datetime.datetime(1682, 8, 15, 16, 55, 52, 727996, tzinfo=<DstTzInfo 'Pacific/Port_Moresby' LMT+9:49:00 STD>)
(Pdb) dt.isoformat()
'1682-08-15T16:55:52.727996+09:49'
(Pdb) DateTime(dt.isoformat())
*** DateTimeError: Unknown time zone in date: 1682-08-15T16:55:52.727996+09:49
(Pdb) tz = get_timezone(dt)
(Pdb) is_valid_timezone(tz)
False
(Pdb) tz
'LMT'
(Pdb) DateTime(dt)
DateTime('1682/08/15 16:55:52.727996 Pacific/Port_Moresby')

Current behavior before PR

Traceback when trying to convert an old date to DateTime

Traceback (innermost last):
  Module ZPublisher.WSGIPublisher, line 176, in transaction_pubevents
  Module ZPublisher.WSGIPublisher, line 385, in publish_module
  Module ZPublisher.WSGIPublisher, line 288, in publish
  Module ZPublisher.mapply, line 85, in mapply
  Module ZPublisher.WSGIPublisher, line 63, in call_object
  Module senaite.core.decorators, line 40, in decorator
  [...]
  Module senaite.core.api.dtime, line 461, in to_localized_time
  Module senaite.core.api.dtime, line 400, in date_to_string
  Module senaite.core.api.dtime, line 142, in to_DT
  Module DateTime.DateTime, line 447, in __init__
  Module DateTime.DateTime, line 775, in _parse_args
DateTimeError: Unknown time zone in date: 1682-08-15T16:55:52.727996+09:49

Desired behavior after PR is merged

datetime is converted to DateTime without error

--
I confirm I have tested this PR thoroughly and coded it according to PEP8
and Plone's Python styleguide standards.

[ramonski]

Thanks!

[ramonski]

Not sure anymore, but I guess it was also related to the timezone, e.g. GMT vs. Etc/GMT or CET etc.