Test on PHP 8

[GrahamCampbell]

There is no upper bound on the PHP version (and too late to add one in 1.x). We should test on PHP 8.0 with the CI.

[dbu]

i wonder if we don't want to add the upper bound anyways. then at least in the future, we are more robust.

[GrahamCampbell]

An upperbound would be a good idea, because almost certainly PHP 9 will have totally deleted the concept of a resource, so things like https://github.com/php-http/message-factory/blob/master/src/StreamFactory.php#L17 need updating with whatever the replacement for fopen's return type is, and related types.