This patch fixes a rare internal error in the
"hypothesis.strategies.datetimes()" strategy, where the implementation
of "allow_imaginary=False" crashed when checking a time during the
skipped hour of a DST transition if the DST offset is negative -
only true of "Europe/Dublin", who we presume have their reasons - and
the "tzinfo" object is a pytz timezone (which predates PEP 495).
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