tevent: version 0.15.0

- remove py2 ifdefs
- python: Safely clear structure members
- the tevent_thread_call_depth API is updated
  in order to allow better tracing.
- add tevent_set_max_debug_level() only and don't
  pass TEVENT_DEBUG_TRACE to tevent_debug() callbacks by default.
- Spelling fixes
- Make use of epoll_create1() for epoll backend
- Optimize overhead in the epoll backend

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>