Add missing module gc3libs.events

Somehow forgotten to add it in commit
d0c1b25

gc3libs/events.py

@@ -0,0 +1,52 @@
+#! /usr/bin/env python
+#
+"""
+Support for communication between parts of code through "events".
+
+This file collects definitions of event classes used across the
+library code. The actual subscription and notification mechanisms
+come from the implementation of the `Observable/Observer pattern`__
+provided by Python's library `generic`__.
+
+.. __: https://en.wikipedia.org/wiki/Observer_pattern
+.. __: http://generic.readthedocs.io/en/latest/event_system.html#event-system
+"""
+# Copyright (C) 2018 University of Zurich
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+__docformat__ = 'reStructuredText'
+
+
+# import these names here, so we can use `from gc3libs.events import
+# subscribe` elsewhere in the code, and leave the dependency on
+# `generic.events` as an implementation detail
+from generic.event import fire as emit, subscribe, unsubscribe
+
+
+# FIXME: rewrite with `attrs` when we drop support for Py2.6!
+class TaskStateChange(object):
+    """
+    Fired when a `Task`:class: execution state changes.
+
+    No guarantee is given as to whether *task* is still in the old
+    *from_state* or has already transitioned to the new *to_state*.
+    """
+
+    __slots__ = ('task', 'from_state', 'to_state')
+
+    def __init__(self, task, from_state, to_state):
+        self.task = task
+        self.from_state = from_state
+        self.to_state = to_state