v1.6.0

• 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)