Sockjs

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- **Breaking change:** Added into the WebSocketTransport the ability
to process multi messages from client (383).
- Added into WebSocketTransport ignoring of empty frames received
from client. (383).
- Added tick after dequeue so heartbeat keeps session live (265).
- Fix race condition during iteration over sessions (217).
- Support Python 3.8.
- Fixed examples of using of SockJS server (264).

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- Sync with aiohttp 3.6 (298)

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- Minor code styling cleanups

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- Support Python 3.7. The minimal available Python version is 3.5.3 (240)

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- Fix heartbeat (214)

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- Fix compatibility with aiohttp 3.0+ again.