[3.15] gh-154352: Fix killing a worker process in regrtest on OpenBSD (GH-154353) (GH-154360)

os.getsid() is only allowed for a process in the same session on
OpenBSD.  Worker processes are started in their own session, so the
failure means that the process group can be killed.
(cherry picked from commit bfd774d)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: 3 people

Lib/test/libregrtest/run_workers.py

@@ -147,9 +147,12 @@ def _kill(self) -> None:
 
         use_killpg = USE_PROCESS_GROUP
         if use_killpg:
-            parent_sid = os.getsid(0)
-            sid = os.getsid(popen.pid)
-            use_killpg = (sid != parent_sid)
+            try:
+                use_killpg = (os.getsid(popen.pid) != os.getsid(0))
+            except PermissionError:
+                # On OpenBSD getsid() is only allowed for a process in the
+                # same session, so the failure means that it is not.
+                use_killpg = True
 
         if use_killpg:
             what = f"{self} process group"