Partially revert "pythongh-87744: fix waitpid race while calling send…

…_signal in asyncio (python#121126)"

This reverts the non-test changes of commit bd473aa.

Lib/asyncio/base_subprocess.py

@@ -1,9 +1,6 @@
 import collections
 import subprocess
 import warnings
-import os
-import signal
-import sys
 
 from . import protocols
 from . import transports
@@ -145,31 +142,17 @@ def _check_proc(self):
         if self._proc is None:
             raise ProcessLookupError()
 
-    if sys.platform == 'win32':
-        def send_signal(self, signal):
-            self._check_proc()
-            self._proc.send_signal(signal)
-
-        def terminate(self):
-            self._check_proc()
-            self._proc.terminate()
-
-        def kill(self):
-            self._check_proc()
-            self._proc.kill()
-    else:
-        def send_signal(self, signal):
-            self._check_proc()
-            try:
-                os.kill(self._proc.pid, signal)
-            except ProcessLookupError:
-                pass
+    def send_signal(self, signal):
+        self._check_proc()
+        self._proc.send_signal(signal)
 
-        def terminate(self):
-            self.send_signal(signal.SIGTERM)
+    def terminate(self):
+        self._check_proc()
+        self._proc.terminate()
 
-        def kill(self):
-            self.send_signal(signal.SIGKILL)
+    def kill(self):
+        self._check_proc()
+        self._proc.kill()
 
     async def _connect_pipes(self, waiter):
         try: