Sockjs

-------------------

- Added argument ``cors_config`` into function ``add_endpoint()``
to support of CORS settings from ``aiohttp_cors``.
- Added arguments ``heartbeat_delay`` and ``disconnect_delay``
into function ``add_endpoint()``.
- Function ``add_endpoint()`` now returns all registered routes.
- Replaced returning instances of error HTTP responses
on raising its as exceptions.
- Changed name of some routes.
- Heartbeat task moved from ``SessionManager`` into ``Session``.
- Methods ``_acquire`` and ``_release`` of ``Sessions`` renamed into
``acquire`` and ``release``.
- Added processing of ``ConnectionError`` in ``StreamingTransport``.
- Changed arguments of handler function. Now handler function must be defined
like ``async def handler(manager, session, msg):``
- Constants:

- FRAME_OPEN
- FRAME_CLOSE
- FRAME_MESSAGE
- FRAME_MESSAGE_BLOB
- FRAME_HEARTBEAT

replaced by ``Frame`` enums with corresponding values.
- Constants:

- MSG_OPEN
- MSG_MESSAGE
- MSG_CLOSE
- MSG_CLOSED

replaced by ``MsgType`` enums with corresponding values.
- Constants:

- STATE_NEW
- STATE_OPEN
- STATE_CLOSING
- STATE_CLOSED

replaced by ``SessionState`` enums with corresponding values.

-------------------

- **Breaking change:** Removed argument ``timeout`` from ``Session.__init__()``
and ``SessionManager.__init__()``.
- **Breaking change:** Argument ``heartbeat`` of ``SessionManager.__init__()``
renamed into ``heartbeat_delay``.
- **Breaking change:** ``Session.registry`` renamed into ``Session.app``.
- **Breaking change:** Deleted method ``SessionManager.route_url()``.
- **Breaking change:** Dropped support of Python < 3.7
- Fixed processing of heartbeats and a session expiration.
- Fixed ping-pong based heartbeats for web-socket connections.
- Added arguments ``heartbeat_delay`` and ``disconnect_delay`` into
``Session.__init__()``.
- Added argument ``disconnect_delay`` into ``SessionManager.__init__()``.

-------------------

- **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).

-------------------

- 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)