Support Python 3.8

[maxbrunet]

Python 3.8.0 has been released on October 14, 2019:
https://www.python.org/downloads/release/python-380/

I have been testing my project against 3.8-dev for a few weeks and it has reported only 1 warning (4 occurrences):

DeprecationWarning: "@coroutine" decorator is deprecated since Python 3.8, use "async def" instead

Full output

/home/travis/virtualenv/python3.8-dev/lib/python3.8/site-packages/mattermostdriver/websocket.py:19

  /home/travis/virtualenv/python3.8-dev/lib/python3.8/site-packages/mattermostdriver/websocket.py:19: DeprecationWarning: "@coroutine" decorator is deprecated since Python 3.8, use "async def" instead

    def connect(self, event_handler):

/home/travis/virtualenv/python3.8-dev/lib/python3.8/site-packages/mattermostdriver/websocket.py:54

  /home/travis/virtualenv/python3.8-dev/lib/python3.8/site-packages/mattermostdriver/websocket.py:54: DeprecationWarning: "@coroutine" decorator is deprecated since Python 3.8, use "async def" instead

    def _start_loop(self, websocket, event_handler):

/home/travis/virtualenv/python3.8-dev/lib/python3.8/site-packages/mattermostdriver/websocket.py:73

  /home/travis/virtualenv/python3.8-dev/lib/python3.8/site-packages/mattermostdriver/websocket.py:73: DeprecationWarning: "@coroutine" decorator is deprecated since Python 3.8, use "async def" instead

    def _authenticate_websocket(self, websocket, event_handler):

/home/travis/virtualenv/python3.8-dev/lib/python3.8/site-packages/mattermostdriver/websocket.py:103

  /home/travis/virtualenv/python3.8-dev/lib/python3.8/site-packages/mattermostdriver/websocket.py:103: DeprecationWarning: "@coroutine" decorator is deprecated since Python 3.8, use "async def" instead

    def _wait_for_message(self, websocket, event_handler):

Documentation:

• https://docs.python.org/3.8/library/asyncio-task.html#asyncio.coroutine
• https://docs.python.org/3.8/reference/compound_stmts.html#coroutine-function-definition

The new syntax async def is new in Python 3.5, meaning Python 3.4 support would have to be dropped to fix these warnings. Python 3.4 having reached EOL on March 18, 2019 (https://python.org/downloads/release/python-3410), I think this should be acceptable.