Whitespace before ':' (E203) should not apply to slices

[FabianNiehaus]

When definining a string slice like this:

my_string[my_string.rindex("/") + 1 : my_string.index(":")]

flake8 will raise violation E203.

However, this should not apply to slices, as mentioned in https://www.python.org/dev/peps/pep-0008/#pet-peeves.

Manually removing the spaces does not work in our case, as code formatting is done by black, which will automatically add whitepspaces around : when used in a slice.

[FabianNiehaus]

Opened this issue in the wrong repo, reposted it on the pycodestyle repo: PyCQA/pycodestyle#373 (comment)