Python-engineio-v3

- better error handling during websocket connection handshake ([commit](https://github.com/miguelgrinberg/python-engineio/commit/f4c49c44a9b83a9fdc286bb38ff3be39b165118b))
- more places where connection shouldn't be reset too quickly ([commit](https://github.com/miguelgrinberg/python-engineio/commit/693b51b2221d59863e2680acbf0f02170bb87a81))

- do not reset connection when ping loop exits ([commit](https://github.com/miguelgrinberg/python-engineio/commit/dafbdb80ffb5eba0522adc14728bb47e13f0ac54))

- never import invalid async drivers ([commit](https://github.com/miguelgrinberg/python-engineio/commit/61b04ea89cf2cc358a40f7854c31859aea8e30d6))

- fixed unreliable unit test ([commit](https://github.com/miguelgrinberg/python-engineio/commit/7e53442afa93d7155c49681050a7aacaaf7222e9))
- Fixes [236](https://github.com/miguelgrinberg/python-socketio/issues/236) ([commit](https://github.com/miguelgrinberg/python-engineio/commit/e6c285882ed023c17e319cf2bd9c3322a524125a))
- fixed handling of queue empty exceptions [88](https://github.com/miguelgrinberg/python-engineio/issues/88) ([commit](https://github.com/miguelgrinberg/python-engineio/commit/bc128c2e3f41a69d855acabdbdbad072f662df92))

- add a grace period of 5 seconds to ping timeout ([commit](https://github.com/miguelgrinberg/python-engineio/commit/b5f15e34ed9ef3a9f75d778c67e9bde4265618a7))
- do not use six in setup.py ([commit](https://github.com/miguelgrinberg/python-engineio/commit/c41bb5d0431c0a5d3c49a98392f530a93fd093c0))
- minor refactor of clients for consistency with servers ([commit](https://github.com/miguelgrinberg/python-engineio/commit/d9c278f326db52da1343c0f7fb4257ff7087e83c))
- minor refactor of the async drivers ([commit](https://github.com/miguelgrinberg/python-engineio/commit/0478110179f91f51c6a7a972b3e284b06c3db2ee))

- unit test reorganization ([commit](https://github.com/miguelgrinberg/python-engineio/commit/3786502ed57920d6b78283bf16150f1711721d38))
- do not block upgrades with high packet traffic [16](https://github.com/miguelgrinberg/python-engineio/issues/16) ([commit](https://github.com/miguelgrinberg/python-engineio/commit/919d8ea639d5190d345b22168d6cbfbdebc421af))
- user session documentation ([commit](https://github.com/miguelgrinberg/python-engineio/commit/af3a0aa7d5b83f8c407f792cb688ef6f983b056b))
- remove python 3.4 from builds ([commit](https://github.com/miguelgrinberg/python-engineio/commit/8c9bbf4132f4cf4082c96d58a6e7270ad7eea0ff))
- user sessions ([commit](https://github.com/miguelgrinberg/python-engineio/commit/561efad215661bfce3d00ffcb5c3290555de8f12))