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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def dtstr_to_datetime(dtstr, to_tz=None, fail_silently=True):
 """ Convert result from datetime_to_dtstr to datetime in timezone UTC0. """

    try:
        dt = datetime.datetime.utcfromtimestamp(int(dtstr, 36) / 1e3)
        if to_tz:
            dt = timezone.make_aware(dt, timezone=pytz.UTC)
            if to_tz != pytz.UTC:
                dt = dt.astimezone(to_tz)
        return dt
    except ValueError, e:
        if not fail_silently:
            raise e
        return None