asyncio.unix_events: Make AbstractChildWatcher abstract, add PidfdChildWatcher (#7412)

tests/stubtest_allowlists/darwin-py310.txt

@@ -3,6 +3,4 @@ _curses.color_pair
 curses.color_pair
 
 # Exists at runtime, but missing from stubs
-asyncio.PidfdChildWatcher
-asyncio.unix_events.PidfdChildWatcher
 mmap.MADV_FREE

tests/stubtest_allowlists/darwin-py39.txt

@@ -1,6 +1,4 @@
 _?curses.A_ITALIC
 
 # Exists at runtime, but missing from stubs
-asyncio.PidfdChildWatcher
-asyncio.unix_events.PidfdChildWatcher
 mmap.MADV_FREE

tests/stubtest_allowlists/linux-py310.txt

@@ -18,8 +18,6 @@ signal.sigwaitinfo
 select.epoll.register
 
 # Exists at runtime, but missing from stubs
-asyncio.PidfdChildWatcher
-asyncio.unix_events.PidfdChildWatcher
 os.EFD_CLOEXEC
 os.EFD_NONBLOCK
 os.EFD_SEMAPHORE

tests/stubtest_allowlists/linux-py39.txt

@@ -2,8 +2,6 @@
 select.epoll.register
 
 # Exists at runtime, but missing from stubs
-asyncio.PidfdChildWatcher
-asyncio.unix_events.PidfdChildWatcher
 os.copy_file_range
 os.pidfd_open
 posix.copy_file_range