Add an exponential backoff to admin client

This retries up to 5 times if we get 502s or 503s from stagecraft.
These errors should be fixed, and there are stories in place to
allow us to investigate them more closely. However, at the moment this
lack of any backoff whatsoever is causing 500s in backdrop.
Remove support for PY3 until backoff is compatible

I have left the version in there as we should be turning it on again as
soon as backoff is good.
Merge pull request #24 from alphagov/add-backoff-to-admin-get

Add an exponential backoff to admin client
Set a custom user agent string

This makes it easier to spot requests from the client. It would be nice
to extend this with the ability for the user of this client to set their
own extension to this user agent.
Add version to admin client user agent

Not ideal, the underlying client should be shared and this should be set
on both.