- Switch to _multibuild to not create buildcycle

- Update to 2.8.0 (2019-02-04)
+ Data updates
* Updated tzdata version to to 2018i.
+ Features
* Added support for EXDATE parameters when parsing rrule
strings. Reported by @mlorant (gh issue #410), fixed by
@nicoe (gh pr #859).
* Added support for sub-minute time zone offsets in Python
3.6+. Fixed by @cssherry (gh issue #582, pr #763)
* Switched the tzoffset, tzstr and gettz caches over to using
weak references, so that the cache expires when no other
references to the original tzinfo objects exist. This cache-
expiry behavior is not guaranteed in the public interface and
may change in the future. To improve performance in the case
where transient references to the same time zones are
repeatedly created but no strong reference is continuously
held, a smaller “strong value” cache was also added. Weak
value cache implemented by @cs-cordero (gh pr #672, #801),
strong cache added by Gökçen Nurlu (gh issue #691, gh pr
#761)
+ Bugfixes
* Added time zone inference when initializing an rrule with a
specified UNTIL but without an explicitly specified DTSTART;
the time zone of the generated DTSTART will now be taken from
the UNTIL rule. Reported by @href (gh issue #652). Fixed by
@absreim (gh pr #693).
* Fixed an issue where parser.parse would raise Decimal-
specific errors instead of a standard ValueError if certain