- 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:** 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:** Dropped support of Python < 3.7

# Conflicts:
#	requirements.txt

…support of CORS settings from ``aiohttp_cors``.

- 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.

…ction ``add_endpoint()``.

- Methods ``_acquire`` and ``_release`` of ``Sessions`` renamed into ``acquire`` and ``release``.

… 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.

- Fixed code style.