#30014: refactor poll-related classes (#1035)

* #30014: refactor poll-related classes so that poll(), epoll() and devpoll() share the same methods for register(), unregister(), close() and select()

* remove unused attribute

* use specific class attributes instead of select.* constants

* have all classes except SelectSelector a _selector attribute

* BaseException -> Exception

* be explicit in defining a close() method only for selectors which have it

* fix AttributeError