v2.0.0

• Feature / BC break: Update to reactphp/http v1.0.0.
  (#45 by @SimonFrings)

• Feature / BC break: Add type declarations and require PHP 7.1+ as a consequence
  (#47 by @SimonFrings, #49 by @clue)

• Use fully qualified class names in documentation.
  (#46 by @SimonFrings)

• Improve test suite and add .gitattributes to exclude dev files from export.
  Prepare PHP 8 support, update to PHPUnit 9 and simplify test matrix.
  (#40 by @andreybolonin, #42 and #44 by @SimonFrings and #48 by @clue)