Explicitly use SelectorEventLoop on Windows

[ghost]

On Windows, Python 3.8's default event loop is now ProactorEventLoop. [1]
But tornado doesn't support ProactorEventLoop. [2]

Should we use SelectorEventLoop explicitly on Windows?

    if os.name == 'nt':
        import asyncio
        asyncio.DefaultEventLoopPolicy = asyncio.WindowsSelectorEventLoopPolicy

FYI, Python 3.8.0's release schedule: [3]

3.8.0 beta 4: Monday, 2019-08-26
3.8.0 candidate 1: Monday, 2019-09-30
3.8.0 candidate 2: Monday, 2019-10-07 (if necessary)
3.8.0 final: Monday, 2019-10-21

[1] https://docs.python.org/3.8/whatsnew/3.8.html#asyncio
[2] https://github.com/tornadoweb/tornado/blob/master/tornado/platform/asyncio.py#L15-L19
[3] https://www.python.org/dev/peps/pep-0569/#id6

[ghost]

In addition, we can catch NotImplementedError exception when invoking these functions, and prompt the user not to use ProactorEventLoop on Windows.
This is an optional idea.

class AbstractEventLoop:
    ....
    def add_reader(self, fd, callback, *args):
        raise NotImplementedError

    def remove_reader(self, fd):
        raise NotImplementedError

    def add_writer(self, fd, callback, *args):
        raise NotImplementedError

    def remove_writer(self, fd):
        raise NotImplementedError
    ....

above code from: https://github.com/python/cpython/blob/v3.8.0b4/Lib/asyncio/events.py#L500-L510

[ploxiln]

see #2608 and #2686

[ghost]

Sorry, close as duplicated.