Added a fallback to using a ThreadPoolExecutor

[ghost]

This enables devices lacking a hardware semaphore implementation such as some Android Devices (looking at the OnePlus3T here!) to still use black.

Fixes #524

[coveralls]

Pull Request Test Coverage Report for Build 748

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.03%) to 95.825%

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Files with Coverage Reduction	New Missed Lines	%
black.py	1	65.33%

Totals
Change from base Build 743:	-0.03%
Covered Lines:	1974
Relevant Lines:	2060

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💛 - Coveralls

[ambv]

Thanks for testing that this fixes the issue for you. This is important to fix but I think we'll go with a "SerialExecutor" instead as described in #524.