Simplify simplehttp, updating to newer python standards

[tt2468]

Description

There's actually no need to run the _run_app() function using a loop we create. Just add an on_startup hook to override the thread pool. Python asyncio automatically shuts down our custom thread pool executor on loop shutdown, so there's no need for any shutdown logic either.

asyncio.to_thread will automatically use the running loop's thread pool executor, which is the one we've created ourselves.

Motivation and Context

I guess my shitty python code from a few years ago is getting deprecation warnings now.

How Has This Been Tested?

Analyzed some logs of mine

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Code cleanup (non-breaking change which makes code smaller or more readable)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[RytoEX]

Use simplehttp: as the commit prefix here.

Is there no need to call loop.close() in this implementation?

[tt2468]

Correct. web.run_app() does its own loop management, so we don't need to do any shutdown. Newer python also handles threadpool executor shut down automatically, too.