DesbyP

Fix false alarm in E203 rule (issue #373, black incompatibility)

19

## Issue #373 Rule E203 checks for extraneous whitespace before a colon. Yet there is a corner case when the colon is used as a "slice" operator. [PEP8](https://www.python.org/dev/peps/pep-0008/#pet-peeves) says: >...