Update server to work with asyncio v3

[nmattia]

There were a couple of breaking API changes in the asyncio v3 upgrade. these changes broke (among others) the max_concurrency support and the app.shutdown() clean shutdown (cancel() should be called on Tasks).

This also drops the asyncio v2/v3 conditional logic and instead just prints a warning if the system is running an unexpected asyncio version.

[nmattia]

I think this should also fix #56

[nmattia]

I just noticed this other fix: https://github.com/belyalov/tinyweb/pull/52