Update black required versions

[akaihola]

There were two issues with Black versions:

1. with current Black, Mypy shouldn't ignore extra call arguments for black.files.gen_python_files() (see Fix gen_python_files usage for black>=21.7b1.dev9 #189), but we have to make sure a recent enough Black is used when running tests
2. Black < 21.5b1 are incompatible with at least to recently added features, so the minimum required Black version had to be bumped

[sourcery-ai]

Sourcery Code Quality Report

Merging this PR leaves code quality unchanged.

Quality metrics	Before	After	Change
Complexity	3.83 ⭐	3.83 ⭐	0.00
Method Length	74.75 🙂	74.75 🙂	0.00
Working memory	6.74 🙂	6.74 🙂	0.00
Quality	75.51% ⭐	75.51% ⭐	0.00%

Other metrics	Before	After	Change
Lines	175	175	0

Changed files	Quality Before	Quality After	Quality Change
src/darker/black_diff.py	75.51% ⭐	75.51% ⭐	0.00%

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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