Cannot install phpunit on PHP version 8.0.7

[ryotsun]

Bug Report

• [x] Yes, I reviewed the contribution guidelines.
• [x] Yes, more specifically, I reviewed the guidelines on how to write clear bug reports.

Encountered an error when I ran composer install I encountered the following error when I tried package installation.

% composer install
No composer.lock file present. Updating dependencies to latest instead of installing from lock file. See https://getcomposer.org/install for more information.
Loading composer repositories with package information
Updating dependencies
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - phpunit/phpunit[6.5.5, ..., 6.5.14] require php ^7.0 -> your php version (8.0.7) does not satisfy that requirement.
    - phpunit/phpunit[7.0.0, ..., 7.5.20] require php ^7.1 -> your php version (8.0.7) does not satisfy that requirement.
    - Root composer.json requires phpunit/phpunit ^6.5.5 || ^7.0.0 -> satisfiable by phpunit/phpunit[6.5.5, ..., 6.5.14, 7.0.0, ..., 7.5.20].

Environment

% wp --info
OS:	Darwin 20.5.0 Darwin Kernel Version 20.5.0: Sat May  8 05:10:31 PDT 2021; root:xnu-7195.121.3~9/RELEASE_ARM64_T8101 x86_64
Shell:	/bin/zsh
PHP binary:	/usr/local/bin/php
PHP version:	8.0.7
php.ini used:	/usr/local/etc/php/8.0/php.ini
MySQL binary:	/usr/local/opt/[email protected]/bin/mysql
MySQL version:	mysql  Ver 14.14 Distrib 5.7.34, for osx10.16 (x86_64) using  EditLine wrapper
SQL modes:	
WP-CLI root dir:	/Users/xxx/wp-cli
WP-CLI vendor dir:	/Users/xxx/wp-cli/vendor
WP_CLI phar path:	
WP-CLI packages dir:	/Users/xxx/.wp-cli/packages/
WP-CLI global config:	
WP-CLI project config:	
WP-CLI version:	2.6.0-alpha

Possible solution Adopt version of phpunit suitable for PHP 8 and above.

diff --git a/composer.json b/composer.json
index 7540d42..2685743 100644
--- a/composer.json
+++ b/composer.json
@@ -14,7 +14,7 @@
     },
     "require-dev": {
         "composer/composer": ">=1.5.6 <1.7.0 || ^1.7.1 || ^2.0.0",
-        "phpunit/phpunit": "^6.5.5 || ^7.0.0",
+        "phpunit/phpunit": "^6.5.5 || ^7.0.0 || ^8.0.0",
         "wp-coding-standards/wpcs": "^0.14.0 || ^1.0.0 || ^2.0.0"
     },
     "autoload": {