aiomysql does not support TLS on Python 3.8 on Windows

Due to the Python 3.8 changing the default event loop to proactor, `start_tls` does not work, therefore you cannot connect to a server using TLS.

As per https://github.com/tornadoweb/tornado/issues/2608 and https://github.com/aio-libs/aiohttp/issues/4536, this limitation should probably be documented somewhere.

The solution, change the event loop policy before the event loop is created.
```py
async def main():
# Do stuff
pass

if __name__ == "__main__":
policy = asyncio.WindowsSelectorEventLoopPolicy()
asyncio.set_event_loop_policy(policy)
asyncio.run(main())
```