fix pylint warning for super init not called on mixin class (no init there)

[s-m-e]

I plead guilty ...

[astrojuanlu]

Rather than "fixing" the warning, this PR ignores the warning. What about actually calling the __init__()? That way we also don't break Python algorithm to traverse the MRO (I think it's called "cooperative multiple inheritance"):

[s-m-e]

The Orbit class inherits from the OrbitCreationMixin class that is designed as a "mixin", i.e. in this case a class that "only" collects a bunch of class methods. The only reason for the mixin to exist was to break the giant Orbit class into two more manageable parts. That's why it does not have a constructor - why should it? Pylint is not aware of this design pattern and yells at me, hence "fix" ;)