Talisker

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* Make gunicorn an optional dependency
* Fix use of werkzeug debugger

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This release includes a fair amount of internal refactoring, and some new features.

* Logs and metrics for timeouts in gunicorn (412)
* Errors are now handled in WSGI, and return text/html/json per Accept header (409)
* Soft timeouts are sent after the request is finished (411)
* Add requests.TaliskerAdapter, a requests transport adapter with loadbalancing and retries (experimental, 405, 408)
* Move logging from gunicorn into WSGI middleware (402)
* Don't use raven's WSGI middleware, do it in our middleware (406)
* All /_status/info/ endpoints support rendering JSON as well as text and HTML (407)
* support rendering json responses as well as text and html (407)
* move the request-id header name to config (401)
* fix requests latency buckets (413)

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* add additional sanitised keys by default, and add config for additional keys. Requires raven 6.4 (392)
* fix parsing of extra['start_time'] being a string in recent raven (390)

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This release fixes an accidental hard dependency on celery that was added in 0.10.0.

The main feature is refactor of Talisker configuration, which now allows for
using a config file as well as environment variables. It also provides
consistant documentation for all configuration in a single place, and a new
talisker.help cli tool, and a new /_status/info/config endpoint for introspection.

* Add documentation for config, including new talisker.help command (386)
* Ensure optional extras are actually optional (385)
* Improve config error handling and logging of errors. (382)
* Add centralised config, with config file support (380)

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* Add companion TestContext.assert_not_log to match assert_log

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* Add assert_log helper to TestContext (377)
* Fixed where some classes where accidentally old-style classes in python 2