Feature: Support PHP 8 (socket address of closed socket should be null).
(#39 by @clue)

Improve test suite and add .gitattributes to exclude dev files from exports.
Run tests on PHPUnit 9, switch to GitHub actions and clean up test suite.
(#30, #31 and #38 by @clue, #34 by @reedy, #35 by @WyriHaximus and #37 by @SimonFrings)