Python-engineio-v3

- _websocket_handler waits on writer even after the socket is closed This patch wakes up the writer with a null message when the socket gets closed ([commit](https://github.com/miguelgrinberg/python-engineio/commit/0c439be734b29b1006b7c6d43f1acc6d2260ed9c)) (thanks **Babu Shanmugam**!)

- Runtime error when websocket is missing from environment ([commit](https://github.com/miguelgrinberg/python-engineio/commit/2ba8a89df6558f088d2cccb541602893929954c0))

- fix python 3.5 build ([commit](https://github.com/miguelgrinberg/python-engineio/commit/bba8f4bb39634b1523e6932f4b6378251dc0d401))
- tolerate payloads in UPGRADE packet [7](https://github.com/miguelgrinberg/python-engineio/issues/7) ([commit](https://github.com/miguelgrinberg/python-engineio/commit/0193b5c9935f9dd56e48610499ea6b528cf09582))

- expose start_background_thread() as a public method ([commit](https://github.com/miguelgrinberg/python-engineio/commit/16aa518d565b7af12c3de07d3416700da5bc99ec))
- Added python 3.5 to the tox build ([commit](https://github.com/miguelgrinberg/python-engineio/commit/5be3c32d2688655891b203aef2d0a31b2b536d6a))

- Correctly end eventlet's websocket connection See miguelgrinberg/flask-socketio[167](https://github.com/miguelgrinberg/python-engineio/issues/167) for the problem this fixes. ([commit](https://github.com/miguelgrinberg/python-engineio/commit/fef4b4739d074cebc741d58e099d8b6459e96112))

- More robust handling of the upgrade exchange ([commit](https://github.com/miguelgrinberg/python-engineio/commit/66b4bc14cb514230799e116a08410e4c3b1deb15))