async def METHOD_NAME(self):
    """
    Authenticates with username and password and on success stores the access token
    and expire time.
    """
    async with aiohttp.ClientSession(loop=asyncio.get_event_loop()) as session:
        params = {'username': self.username, 'password': self.password}
        async with session.post(urljoin(self.http_host, '/access_token/'), data=params) as response:
            data = await self.parse_answer(response)
            self.access_token = data['access_token']
            self.token_expire_time = time.time() + round(data['expires_in'] / 2)