Mypy & Black 21.x compatibility

[akaihola]

Enable implicit re-export from Black

Black 21.x split the code base into modules. Symbols are not re-exported in the main package, so we need this Mypy directive in order to pass Mypy checks when Darker imports from Black.

We can't yet import directly from the Black submodules since we want to keep supporting Black 20.x at least for some time still.

[sourcery-ai]

Sourcery Code Quality Report

Merging this PR leaves code quality unchanged.

Quality metrics	Before	After	Change
Complexity	2.00 ⭐	2.00 ⭐	0.00
Method Length	65.67 🙂	65.67 🙂	0.00
Working memory	7.17 🙂	7.17 🙂	0.00
Quality	77.90% ⭐	77.90% ⭐	0.00%

Other metrics	Before	After	Change
Lines	122	122	0

Changed files	Quality Before	Quality After	Quality Change
src/darker/black_diff.py	77.90% ⭐	77.90% ⭐	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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