criu-ns: Add support for older Python version in CI

These changes remove and update the changes introduced in
7177938e60 in favor of the
Python version in CI.

os.waitstatus_to_exitcode() function appeared in Python 3.9

Related to: #1909

Signed-off-by: Dhanuka Warusadura <csx@tuta.io>

scripts/criu-ns

@@ -71,7 +71,19 @@ def _wait_for_process_status(criu_pid):
         try:
             (pid, status) = os.wait()
             if pid == criu_pid:
-                return os.waitstatus_to_exitcode(status)
+                # The following code block is based on
+                # os.waitstatus_to_exitcode() introduced in Python 3.9
+                # and we implement this for comparability with older
+                # versions of Python.
+                if os.WIFSIGNALED(status):
+                    return os.WTERMSIG(status)
+                elif os.WIFEXITED(status):
+                    return os.WEXITSTATUS(status)
+                elif os.WIFSTOPPED(status):
+                    return os.WSTOPSIG(status)
+                else:
+                    raise Exception("CRIU was terminated by an "
+                                    "unidentified reason")
         except OSError:
             return -251