Thanks for the PR @agronholm.

+1, loop.connect_accepted_socket() seems to me like it should be included in AbstractEventLoop, as it's important to have for asyncio interoperability purposes when used in addition to other libraries for handling connections. I think you are correct @agronholm that it was simply missed, but I'll wait on approval from @1st1 and/or @asvetlov to confirm this prior to merging.

Also, this should include an update to the What's New document for 3.10, in the "Porting to Python 3.10 > Changes in the Python API" sub-section since it's adding a method to an ABC. See the 3.9 What's New for an example: https://docs.python.org/3.9/whatsnew/3.9.html#changes-in-the-python-api (since the section doesn't exist yet in the 3.10 version).