Fix compatibility with Black >= 21.7b1.dev9

[akaihola]

Duplicate of #189.

[sourcery-ai]

Sourcery Code Quality Report

❌  Merging this PR will decrease code quality in the affected files by 2.79%.

Quality metrics	Before	After	Change
Complexity	3.91 ⭐	3.85 ⭐	-0.06 👍
Method Length	68.75 🙂	75.25 🙂	6.50 👎
Working memory	6.59 🙂	7.59 🙂	1.00 👎
Quality	75.95% ⭐	73.16% 🙂	-2.79% 👎

Other metrics	Before	After	Change
Lines	171	180	9

Changed files	Quality Before	Quality After	Quality Change
src/darker/black_diff.py	75.95% ⭐	73.16% 🙂	-2.79% 👎

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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