Fix gen_python_files usage for black>=21.7b1.dev9

[flying-sheep]

This should fix the tests and be backwards compatible

[akaihola]

As noted by @flying-sheep, I already had a fix for this in #183. The difference is that my solution looks at black.__version__ and this one inspects the actual signature of the function. Is this just a matter of taste or do you see pros and cons in each approach?

[akaihola]

Could you add a change log entry similar to the one in the alternative pull request?

[akaihola]

You can see a couple of comments in the review. I'm starting to think I like the inspect approach better than looking at the Black version.

[sourcery-ai]

Sourcery Code Quality Report

❌  Merging this PR will decrease code quality in the affected files by 0.44%.

Quality metrics	Before	After	Change
Complexity	3.91 ⭐	3.83 ⭐	-0.08 👍
Method Length	68.75 🙂	74.75 🙂	6.00 👎
Working memory	6.59 🙂	6.74 🙂	0.15 👎
Quality	75.95% ⭐	75.51% ⭐	-0.44% 👎

Other metrics	Before	After	Change
Lines	171	175	4

Changed files	Quality Before	Quality After	Quality Change
src/darker/black_diff.py	75.95% ⭐	75.51% ⭐	-0.44% 👎

Here are some functions in these files that still need a tune-up:

File	Function	Complexity	Length	Working Memory	Quality	Recommendation

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[flying-sheep]

Is this just a matter of taste or do you see pros and cons in each approach?

I think the approach here is simpler in two ways:

1. it uses less code
2. it checks directly for the difference that has to be handled as opposed to taking an indirect route over the version

but ultimately it’s a matter of taste as both solutions are likely to be robust.

[akaihola]

Thanks @flying-sheep for the improved solution!