Late static binding

[Jules Pietri]

Hi there,

I'm actually working on a "functional tests helper", in a way a wrapper for this very useful library that you made and shared.

To be more flexible, I would like to first extend your Assert class to override some methods, among them valueToString() could use a custom formatter and since it is protected I thought it was an opening for what I'm trying to do.

The problem is that all the static calls in that class use self, so to use an overridden valueToString I have to override all the methods...

Would you accept a PR that changes all the self keyword by static to allow such usage?

If not, may I ask what's the point of having protected static methods instead of private?

Thanks,

Jules