Add trailing comma when a single import doesn't fit on a line.

[zsol]

Fixes #250.

[coveralls]

Pull Request Test Coverage Report for Build 710

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.003%) to 96.113%

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Files with Coverage Reduction	New Missed Lines	%
tests/test_black.py	1	97.77%

Totals
Change from base Build 708:	0.003%
Covered Lines:	2720
Relevant Lines:	2830

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💛 - Coveralls

[zsol]

@ambv I'd appreciate your eyes on this. Not sure if there's a better place for the logic here

[ambv]

Don't do this. There is one function in black that is essentially a giant if statement and that's whitespace. Everything else should conform to C901. I'll merge this as is and fix forward.