Aiorequestful

Changed
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* Update upstream dependency versions

Fixed
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* Typing in :py:class:`.RequestHandler`
* Added missing python-dateutil dependency

Changed
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* Moved :py:class:`.RequestHandler` to base of package in request.py
* Moved all :py:class:`.Timer` implementations to base of package in timer.py
* Moved all request exceptions to base of package in exception.py

Documentation
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* Finalise writing guides

Added
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* Raise an exception on :py:meth:`.RequestHandler.request` when called and the session is closed.

Documentation
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* Expand README to complete all placeholder sections

Changed
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* Rename AuthResponseHandler to :py:class:`.AuthResponse`.
Implements MutableMapping to allow handling of response on the object directly.
* Rename AuthResponseTester to :py:class:`.AuthTester`.
:py:meth:`.AuthTester.test` now only requires the :py:class:`.AuthResponse` for input.

Fixed
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* Removed bad exception condition on retry timer in :py:class:`.RequestHandler`

Fixed
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* Bug when awaiting :py:class:`.Timer.