Support testing against PHP 8 and 8.1.

[dcr-stripe]

Summary

This PR:

• Runs CI for PHP 8.0 and PHP 8.1.
• Changes composer.json to require (as a devDependency) EITHER phpunit 5.7, which is compatible with PHP <=7.2; OR phpunit 9.0, which is compatible with PHP 8.
  • Some functions in phpunit 9.0 don't exist in phpunit 5.7. To keep our test suite compatible with both, we extend the PHPUnit TestCase class and add some "shims" over top of them.
• Fixes the flaky StripeTelemetryTest
• Updates the php-cs-fixer task to run on PHP8
  • Towards that end, upgrade php-cs-fixer to the recent version 3.2.1 (|| 2.17.1 is provided as a fallback so composer install doesn't break on old versions of PHP)
  • Make small formatting/style changes associated with that upgrade.

fixes #1200 #1199

[richardm-stripe]

@dcr-stripe will you take a look at this?

[dcr-stripe]

This looks good to me overall! Thanks for all your hard work on this Richard! 🎉

(I can't approve since it's my own PR)

[dcr-stripe]

Should we run this on 8.1 as well?

[richardm-stripe]

Yes, I have that in a forthcoming PR. There are some warnings I need to address first.

[dcr-stripe]

Confirming this was just a stale require?

[richardm-stripe]

Good call. I assumed this too, but I was wrong. We actually use it in StripeMock.php which apparently attempts to boot up stripe-mock if you aren't already running it?

In our CI we always boot up stripe-mock in a separate step, and I always boot stripe-mock up manually, so I went ahead and removed this file, and now the devDependency is unused.

[dcr-stripe]

LGTM - good find!

[hivokas]

This is awesome, thanks for your work!