* add some doc about wtf this thing does
* pin Twisted to < 18.4
* add explicit dep on six (fixes #3089)

* When creating a new event, cap its depth to 2^63 - 1
* When receiving events, reject any without a sensible depth

As per https://docs.google.com/document/d/1I3fi2S-XnpO45qrpCsowZv8P8dHcNZ4fsBsbOW7KABI

Fix media admin APIs

Fix quarantine media admin API

fix formatting on line break

This reverts commit 08b29d45.

Reject events which have lots of prev_events

Make tests py3 compatible

Use six.itervalues in some places

add __bool__ alias to __nonzero__ methods

Add some more variables to the unittest config

Replace Queue with six.moves.queue

Use str(e) instead of e.message

Clarify that SRV may not point to a CNAME

Document contrib directory

Refactor ResponseCache usage

Refactor store.have_events

It turns out that most of the time we were calling have_events, we were only
using half of the result. Replace have_events with have_seen_events and
get_rejection_reasons, so that we can see what's going on a bit more clearly.

Avoid creating events with huge numbers of prev_events

... this should protect us from being dossed by people making silly events
(deliberately or otherwise)

In most cases, we limit the number of prev_events for a given event to 10
events. This fixes a particular code path which created events with huge
numbers of prev_events.

and a six.range change which I missed the last time

Signed-off-by: Adrian Tschira <nota@notafile.com>

This is a mixed commit that fixes various small issues

 * print parentheses
 * 01 is invalid syntax (it was octal in py2)
 * [x for i in 1, 2] is invalid syntax
 * six moves

Signed-off-by: Adrian Tschira <nota@notafile.com>

Signed-off-by: Adrian Tschira <nota@notafile.com>