py3: catch ChildProcessError instead of checking errno == ECHILD

mercurial/commandserver.py

@@ -693,9 +693,7 b' class unixforkingservice:'
         while self._workerpids:
             try:
                 pid, _status = os.waitpid(-1, options)
-            except OSError as inst:
-                if inst.errno != errno.ECHILD:
-                    raise
+            except ChildProcessError:
                 # no child processes at all (reaped by other waitpid()?)
                 self._workerpids.clear()
                 return

mercurial/worker.py

@@ -186,13 +186,10 b' def _posixworker(ui, func, staticargs, a'
             p = st = 0
             try:
                 p, st = os.waitpid(pid, (0 if blocking else os.WNOHANG))
-            except OSError as e:
-                if e.errno == errno.ECHILD:
+            except ChildProcessError:
                     # child would already be reaped, but pids yet been
                     # updated (maybe interrupted just after waitpid)
                     pids.discard(pid)
-                else:
-                    raise
             if not p:
                 # skip subsequent steps, because child process should
                 # be still running in this case