Add support for black >= 22.1.0

[Mystic-Mirage]

Black changed find_project_root behavior. Now it returns a tuple.
Let's use find_pyproject_toml instead.

[sourcery-ai]

Sourcery Code Quality Report

❌  Merging this PR will decrease code quality in the affected files by 0.38%.

Quality metrics	Before	After	Change
Complexity	1.49 ⭐	1.49 ⭐	0.00
Method Length	27.78 ⭐	27.78 ⭐	0.00
Working memory	4.60 ⭐	4.76 ⭐	0.16 👎
Quality	89.49% ⭐	89.11% ⭐	-0.38% 👎

Other metrics	Before	After	Change
Lines	178	178	0

Changed files	Quality Before	Quality After	Quality Change
src/darker/config.py	91.55% ⭐	91.59% ⭐	0.04% 👍
src/darker/import_sorting.py	84.42% ⭐	83.00% ⭐	-1.42% 👎

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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[Mystic-Mirage]

Duplicates #267

[akaihola]

I took yet another approach to this in #270 by accepting both the old and the new behavior, and encapsulating that in a "Black compatibility" module.