1.x php 8 compatibility

[mbolli]

This is an ongoing pull request for issue #21.

• I had to use dev-master of the juliangut/mapping repo
• To fix the tests, I had to add a dummy AttributeDriver (would be nice to make it work as well)
• I had to disable phpstan in the qa action, as it wanted to enforce typed properties but PHP 7.0 doesn't support these yet (solution could be to use different phpstan rules or to raise the minimum PHP 7 version)

[mbolli]

It already appears to be working like this with my PHP 8.0.9.

@juliangut

• Do you already have an AttributeDriver for the 2.x branch which you can backport?
• Can you make a release of juliangut/mapping@b721c24 ?

[mbolli]

Hey @juliangut sorry for the bump but could I get a quick feedback?

[mbolli]

@juliangut
I guess you have enough to do as it is, I just want to recall this to your mind (see my questions in #22 (comment)). This would fix issue #21.