gh-95649: Document that asyncio contains uvloop code (#107536)

Some of the asyncio SSL changes in GH-31275 [1] were taken from
v0.16.0 of the uvloop project [2]. In order to comply with the MIT
license, we need to just need to document the copyright information.

[1]: https://github.com/python/cpython/pull/31275
[2]: https://github.com/MagicStack/uvloop/tree/v0.16.0