Fix CVE-2015-8688, and a few bugfixes.

slixmpp is vulnerable to roster push attacks as described by Daniel
Gultsch at https://gultsch.de/gajim_roster_push_and_message_interception.html.

(CVE-2015-8688)

(probably a past typo)

(typo)

Fix a few bugs along with the testsuite, and remove the asyncio loop
monkeypatch hack.

This allowed us to schedule events in-order later in the event loop, but
was detrimental to using other event loops and debugging.

They were using deprecated (and-removed) style.

Re-add relevant stanza methods, broken in 7cd1cf32

it breaks assumptions on the type of the value

Try to not append slixmpp stanzas to ElementTree objects.

Broken since 125336ae due to unforeseen consequences of a variable
removal.

Broken since 3a9b45e4 because of an overzealous cleanup.

Previously, trying to get a text node with a lang which is different
from the one we specified would return nothing, which means e.g. a
message would be ignored because its body is of lang 'fr' when we setup
slixmpp to prefer 'en'. We want to return something when there is an
available, valid content in a different language.

Fix #3165, we don’t need to use the long form to get the superobject in
our supported python versions.

Thanks tchiroux for raising the issue and providing the fix as well.

broken in 3a9b45e4

This gains about 1/8th of the time spent in __getitem__.