Bump phpunit/phpunit from 7.5.1 to 8.1.2

Bumps [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) from 7.5.1 to 8.1.2.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/master/ChangeLog-8.1.md)
- [Commits](sebastianbergmann/phpunit@7.5.1...8.1.2)

Signed-off-by: dependabot[bot] <support@dependabot.com>

composer.json

@@ -14,7 +14,7 @@
     "require": {
         "php": "^7.3",
         "amphp/amp": "^2.1.1",
-        "phpunit/phpunit": "^7.5.1"
+        "phpunit/phpunit": "^7.5.1 || ^8.0.0"
     },
     "require-dev": {
         "harmonyio/php-codesniffer-ruleset": "1.0.0-rc1",