EpollPoller registers with mask EPOLLIN, but filters as if also expecting EPOLLHUP

[moreati]

EpollPoller registers with the mask EPOLLIN | EPOLLOUT

https://github.com/mitogen-hq/mitogen/blob/bb9c51b3e9cc39fceddd55578bb89680fa4e1acc/mitogen/parent.py#L1044-L1047

but when looping over a poll result it filters read file descriptors (self._rfds) on EPOLLIN | EPOLLHUP

https://github.com/mitogen-hq/mitogen/blob/bb9c51b3e9cc39fceddd55578bb89680fa4e1acc/mitogen/parent.py#L1080-L1081 https://github.com/mitogen-hq/mitogen/blob/bb9c51b3e9cc39fceddd55578bb89680fa4e1acc/mitogen/parent.py#L1089-L1090

I'm not familiar with epoll() so this may be working as intended, but it feels like a bug. Noticed while investigating #957.