Add events property to boto3.session.Session

[kyleknap]

Before the only way to hook into the 'creating-client-class' and 'creating-resource-class' events was to first create a botocore session and pass that to the instantiation of the boto3.session.Session object and register using the botocore session. This gives users access to the event system through the boto3.session.Session object much like you have access to it via the client.meta.events property.

cc @jamesls @mtdowling @rayluo

[jamesls]

Looks good, just wondering about the property name. I would expect something called events to return a list of events. What about event_emitter?

[kyleknap]

@jamesls I chose events to be consistent with the client's events attribute. I agree the event_emitter is much more explicit/clear and probably in hindsight we should have used that for the client. Would it make it confusing to call it events on the client but event_emitter on the session when they are the same thing?

[jamesls]

Yeah that's too bad. Let's keep it with .events so we're at least consistent.

[mtdowling]

Alternatively, we could add event_emitter to client and deprecate events.

[kyleknap]

We talked about this offline. We are going to stick with events. Merging.