Fix compatibility with Python 3.13

[marmarek]

_UnixSelectorEventLoop in Python 3.13 expect _unix_server_sockets dict, add one. It's later used (among others) in _stop_serving(), but not in close() directly, so just adding attribute is enough.

The related CPython change: 74b868f636a "gh-111246: Remove listening Unix socket on close (#111483)"

PR Checklist:

• [x] All new features have been tested
• [x] All new features have been documented
• [x] I have read the CONTRIBUTING.md file
• [x] I will abide by the code of conduct