Add Server.close_clients() and Server.abort_clients()

[ybai08]

Fixes #683.

Python 3.13 added Server.close_clients() and Server.abort_clients() to asyncio, but uvloop.loop.Server doesn't have them: AttributeError: 'uvloop.loop.Server' object has no attribute 'close_clients'.

The server now keeps its client transports in a WeakSet (as asyncio's Server._clients does), next to the existing _active_count. _attach() / _detach() take the transport, and the two new methods call close() / abort() on a snapshot of that set. The methods exist on every Python version uvloop supports, not just 3.13+. wait_closed() keeps its current behaviour.

Tests: test_server_close_clients / test_server_abort_clients in tests/test_tcp.py start a server with three connected clients and call srv.close(). They check that the connections stay open, then call the new method and check that every client sees EOF (or a reset for abort_clients) and every handler finishes. The asyncio variants are skipped before 3.13.

Testing (macOS arm64, built with --use-system-libuv against Homebrew libuv 1.53.0):

• With the source files reverted to master, both Test_UV_TCP tests fail on 3.13: AttributeError: 'uvloop.loop.Server' object has no attribute 'close_clients' (and the same for abort_clients). The Test_AIO_TCP variants pass.
• With the change, all four pass on 3.13, and the uvloop ones pass on 3.12.
• The suite was run with python -m unittest discover tests on 3.12 and 3.13, and with PYTHONASYNCIODEBUG=1 on 3.13: 531 tests each. The only failure is test_fs_event_rename (FSEvents reports CHANGE instead of RENAME on this machine). It fails the same way in a tree without this change.
• flake8 passes.

Not run: Linux, the free-threaded build, the bundled-libuv build, and benchmarks. The accept path now does one WeakSet.add() and one discard() per connection.