docs: def exitcode(self):
          """
          Process exit code. :const:`0` when process exited successfully,
          positive number when exception was occurred, negative number when
          process was signaled and :data:`None` when process has not exited
          yet.
          """
          if self._process is None:
              raise ProcessError(
                  "Process '%s' has not been started yet" % self.name)
          return self._process.exitcode