Update isort to follow pyi (stub files) standards, in similar fashion to black

[timothycrosley]

Currently, isort is unaware of the differences between the established coding standards for .py files and .pyi files, treating them the same. This ticket represents updating isort to be aware of these differences, in particular for .pyi files and follow the appropriate standard for the given file type. https://github.com/python/typeshed/blob/master/CONTRIBUTING.md#stub-file-coding-style provides a good initial intro to the .pyi specific standards.

In particular, any resolution to this ticket should resolve the incompatibility with Black outlined here: psf/black#837

[timothycrosley]

Fixed in release 4.3.19