Run WindowsSelectorEventLoop event loop explicit on windows and py3.

[p0psicles]

• Required from python3.8.

• PR is based on the DEVELOP branch
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[sharkykh]

FYI this will throw an exception for Python 3.5 and 3.6 as WindowsSelectorEventLoopPolicy is only available on 3.7.0 and newer:
https://docs.python.org/3.7/whatsnew/changelog.html#id98

bpo-33792: Add asyncio.WindowsSelectorEventLoopPolicy and asyncio.WindowsProactorEventLoopPolicy.

More info from Tornado:

On Windows, Tornado requires the WindowsSelectorEventLoop. This is
the default in Python 3.7 and older, but Python 3.8 defaults to an
event loop that is not compatible with Tornado. Applications that use
Tornado on Windows with Python 3.8 must call
asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())
at the beginning of their main file/function.

I suggest

# We need to set the WindowsSelectorEventLoop event loop on python 3 (3.8 and higher) running on windows
if sys.platform == 'win32':
    try:
        asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())
    except AttributeError:  # Only available since Python 3.7.0
        pass

[medariox]

@sharkykh
Good catch! Mind opening a PR?

[p0psicles]

From what is Read was that it was always available below 3.8. But from 3.8 needed to be set explicitly.

Good to know.
Tnx

[sharkykh]

@medariox Sure #7942
@p0psicles It was the default up until 3.8. Tornado says that since 3.8 the default policy on Windows is not compatible with Tornado.