Explicitly configure the aioapns event loop so proxying works when configured

[devonh]

When the aioapns dependency was updated, they removed the ability to inject an event loop. This change sets the event loop manually after creating the apns client.

We use a custom event loop in this case in order to override create_connection when a proxy is configured to force all traffic over the proxy.

Fixes https://github.com/element-hq/backend-internal/issues/22