[FIX] bus: fix import to not rely in implicit sub-module imports

Also, define `Event` attribute in `__init__` where it belongs.

Part-of: odoo/odoo#98081

addons/bus/models/bus.py

@@ -99,6 +99,7 @@ class ImDispatch(object):
     def __init__(self):
         self.channels = {}
         self.started = False
+        self.Event = None
 
     def poll(self, dbname, channels, last, options=None, timeout=TIMEOUT):
         if options is None:
@@ -181,7 +182,7 @@ class ImDispatch(object):
     def start(self):
         if odoo.evented:
             # gevent mode
-            import gevent
+            import gevent.event  # pylint: disable=import-outside-toplevel
             self.Event = gevent.event.Event
             gevent.spawn(self.run)
         else: