Replace deprecated asyncio.get_child_watcher()

Use PidfdChildWatcher if os.pidfd_open is available and
otherwise use ThreadedChildWatcher which should work in any case.

Fixes neovim#583

pynvim/msgpack_rpc/event_loop/asyncio.py

@@ -188,10 +188,34 @@ async def connect_stdout():
 
     @override
     def _connect_child(self, argv: List[str]) -> None:
-        if os.name != 'nt':
-            # see #238, #241
-            self._child_watcher = asyncio.get_child_watcher()
-            self._child_watcher.attach_loop(self._loop)
+        def can_use_pidfd():
+            # Unix system without pidfd_open
+            if not hasattr(os, 'pidfd_open'):
+                return False
+            try:
+                pid = os.getpid()
+                os.close(os.pidfd_open(pid, 0))
+            except OSError:
+                # blocked by security policy like SECCOMP
+                return False
+            return True
+
+        def get_child_watcher():
+            if hasattr(os, "pidfd_open"):
+                try:
+                    from asyncio.unix_events import PidfdChildWatcher
+                    if can_use_pidfd():
+                        return PidfdChildWatcher()
+                except ImportError:
+                    pass
+
+            try:
+                from asyncio.unix_events import ThreadedChildWatcher
+                return ThreadedChildWatcher()
+            except ImportError:
+                pass
+
+            return asyncio.get_child_watcher()
 
         async def create_subprocess():
             transport: asyncio.SubprocessTransport  # type: ignore
@@ -200,6 +224,12 @@ async def create_subprocess():
             pid = transport.get_pid()
             debug("child subprocess_exec successful, PID = %s", pid)
 
+            if os.name != 'nt':
+                watcher = get_child_watcher()
+
+                watcher.attach_loop(self._loop)
+                self._child_watcher = watcher
+
             self._transport = cast(asyncio.WriteTransport,
                                    transport.get_pipe_transport(0))  # stdin
             self._protocol = protocol

tox.ini

@@ -4,7 +4,7 @@
 [tox]
 min_version = 4.0
 envlist =
-  py{37,38,39,310,311,312}-asyncio
+  py{37,38,39,310,311,312,313}-asyncio
   checkqa
 skip_missing_interpreters =
   true
@@ -18,6 +18,7 @@ python =
   3.10: py310
   3.11: py311
   3.12: py312
+  3.13: py313
   pypy3: pypy3
 
 [testenv]