v1.6.0

• Feature: Forward compatibility with react/promise v3.
  (#37 by @WyriHaximus)

• Improve test suite and add .gitattributes to exclude dev files from exports.
  Run tests on PHPUnit 9 and PHP 7.4 and clean up test suite.
  (#38 by @WyriHaximus, #39 by @reedy, #41 by @clue and #42 by @SimonFrings)