Documentation linting

[Emvlt]

Opening a pull request for improved documentation and linting.

This is quite tedious, as a lot of errors come from line-too-long, trailing-whitespaces (thanks Black!), f-strings encouraged used and unnecessary if/else conditions, but:

• It really helps for cleaning-up: i found that we had forgotten to remove the __array_priority__ attribute of the LinearSpace and Operator and that several operations still relied on it behind our back.
• It makes it clear that function signature between Parent and Children class is not consistent.
• highlights the discrepancy between the new sparse module and the old COO-only sparse class (mostly used in pspace_ops)
• shows that some functions could be broken down into smaller

[leftaroundabout]

I'm generally not a fan of linting, and particularly not of Black which in many cases makes the code less readable.

As I see it, there are two kinds of changes here that are actually worthwhile:

• Changing .format to f-strings
• Removing old-style super(ClassName, self) arguments

The rest of the changes are about 60% unnecessary shuffling around code that doesn't really make a difference, 20% mildly annoying tearing of layout, 5% really bad messing up the layout, and 15% mild improvement of readability.

Just my opinion.

[leftaroundabout]

Would it be possible to run Black to only apply the .format and super changes?