make @parametric work with classes with arbitrary metaclasses

[PhilipVinc]

I am experimenting with https://github.com/cgarciae/treeo and plum, and found out that since both inject meta classes they don't work together.

This PR makes plum work with classes which already have a meta class by injecting CovariantMeta as a superclass of the existing meta class, and by redefining __call__ (essentially the constructor) to infer the parameters before calling into the existing class.

I'm surprised this was so easy to implement!

[coveralls]

Coverage remained the same at 100.0% when pulling a0df7ac on PhilipVinc:pv/parametricmeta into b521bad on wesselb:master.

[wesselb]

Thank you, @PhilipVinc!! This really is excellent—a strict improvement. Merging this now and releasing a new version. :)

[wesselb]

Actually, you've still marked this as a draft. The PR looks complete to me. Are you happy to have this merged?

[PhilipVinc]

Yeah, I think I wanted to add a test to check for what the fix issue in __init_subclass__ commit fixed.
I found out about it in a very exotic case that arises when you combine @parametric with classes that auto-register themselves with jax, but it's a real issue of plum,.

I'll add a test soon

[wesselb]

The contents of this PR will be included in #68, so closing this.