Channels

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* Bumped msgpack to 0.6

* Enforced Python 3.6 and up because 3.5 is too unreliable.

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* Fix memory leaks with receive_buffer

* Prevent double-locking problems with cancelled tasks

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* Adds compatibility with Python 3.8.

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* Adjusted ``AsgiHandler`` HTTP body handling to use a spooled temporary file,
rather than reading the whole request body into memory.

As a result, ``AsgiRequest.__init__()`` is adjusted to expect a file-like
``stream``, rather than the whole ``body`` as bytes. Test cases instantiating
requests directly will likely need to be updated to wrap the provided body
in, e.g., `io.BytesIO`.

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* Fixed a bug in group_send where it would not work if channel_capacity was set

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* Updated requirements for ASGI v3 and Daphne 2.3.