[WIP] Allow a single blank line between class and function definitions

[sersorrel]

Fixes #619.

However:

• it "fixes" it by allowing up to one blank line between class and def; it won't choose one over the other (nevertheless, I believe Black already does this for blank lines inside functions, so it's not totally without precedent 🙂 – in particular, both of the strict behaviours have been tried in the past (before Class new line between docstrings / vars / methods #219, one blank line was enforced; now, Black removes the blank line), and both have been determined to be suboptimal)
• one test is currently failing due to the above
• there are no new tests specifically for this behaviour (is there documentation on how to write a test, or do I have to copy-paste an existing one?)
• I don't know if the way I've implemented this is actually sensible

There are several cases that weren't covered in the aforementioned issue; for example, should the following (currently rejected) be accepted? Clearly in this case it's worse than the version with no blank line, but if either class had more definitions I think it would make sense:

class C:

    class Inner:
        pass

(which I guess would lead to something like "if the previous line is a class, and this line is a class/function/decorator, allow 0 or 1 leading blank lines"? should this also apply when the previous line is a def?)

tldr: I kind of feel like this is a bit of a hack when a proper solution would be more widely applicable, see #450 (comment). (at the same time, this is about the only irritation I run into on a regular basis, so I don't want to get in the way of merging this too much!)

[cooperlees]

This PR has become very stale. Please reopen if you wish to finish it.