Avoid looking at __name__ attribute in black.linegen.run_transformer

[ichard26]

Right the transformer code looks at the transformer's __name__ attribute when deciding to return early:

black/src/black/linegen.py

Lines 964 to 967 in ef7c45f

	if (
	transform.__name__ != "rhs"
	or not line.bracket_tracker.invisible
	or any(bracket.value for bracket in line.bracket_tracker.invisible)

This is kinda hacky and becomes quite a problem when integrating mypyc since nested functions don't keep that attribute due a bug (mypyc/mypyc#884). The current workaround is to dynamtically create a wrapping class at runtime (via type() mind you 🙃) which do keep their __name__ attribute. Let's avoid this entire situation if feasible ^^

Additional context:

I guess it would be a bigger refactor but I'd like it better if we just stopped using __name__.

by @JelleZijlstra in #2431 (comment)

That's a bit hacky; maybe we should turn the Transformer type into a pair instead to store this extra information.

by @JelleZijlstra in #1621 (comment)