PHP8 compatibility update

[kohlerdominik]

Replaces #106

[kohlerdominik]

@sprain do you know why Travis does not run? Do you have to start it manually?

[sprain]

My TravisCI account is out of OSS credits. I need to have a look and get a better understanding of how they handle this.
Builds have been temporarily disabled for private and public repositories due to a negative credit balance

[kohlerdominik]

I'm not familiar with travis. But Github Actions does a great job, and is quite easy to use, too.
I can provide a github actions PR, if you're interested.

[sprain]

I can provide a github actions PR, if you're interested.

I would be interested :) Thanks a lot!

Travis recently changed some stuff and I did not yet get to catch up on all the news. But it seems to be a pain for oss projects. https://www.jeffgeerling.com/blog/2020/travis-cis-new-pricing-plan-threw-wrench-my-open-source-works

[sprain]

See #126