Upgrade Peewee to latest 3.x

This requires a number of small changes in the data model code, as well as additional testing.

buildman/jobutil/buildjob.py

@@ -194,12 +194,19 @@ class BuildJobNotifier(object):
     with UseThenDisconnect(app.config):
       tags = self.build_config.get('docker_tags', ['latest'])
       trigger = self.repo_build.trigger
+
+      # TODO(bison): This is weird hack.  Figure this out.
+      if trigger is not None and trigger.id is not None:
+        trigger_kind = trigger.service.name
+      else:
+        trigger_kind = None
+
       event_data = {
         'build_id': self.repo_build.uuid,
         'build_name': self.repo_build.display_name,
         'docker_tags': tags,
         'trigger_id': trigger.uuid if trigger is not None else None,
-        'trigger_kind': trigger.service.name if trigger is not None else None,
+        'trigger_kind': trigger_kind,
         'trigger_metadata': self.build_config.get('trigger_metadata', {})
       }