Tests: stop passing non-fixed-offset pytz.timezone() to datetime.datetime()

Passing pytz.timezone objects directly to datetime.datetime is not recommended by pytz upstream, and has probably never worked if they are non-constant-offset.

Recent changes to tzdata (see bug 1084190, but please do not close it as it affects several packages) exposed that a dateparser test currently does this. (In the reference, i.e. dateparser isn't actually broken for users but will FTBFS.) This replaces the reference with a plain datetime.timezone.