support backslashed multiline imports

[mnikulin]

I'm not sure my changes are suitable for the master, but i need autoimport for non black-formatted pep8 code with backslashed multiline imports.

Checklist

• [ ] Add test cases to all the changes you introduce
• [ ] Update the documentation for the changes

[coveralls]

Pull Request Test Coverage Report for Build 2612257223

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 96.262%

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Totals
Change from base Build 2479696016:	0.01%
Covered Lines:	309
Relevant Lines:	321

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💛 - Coveralls