Black breaks flake8's E203 by adding space in slicing expression

[choucavalier]

Describe the bug A clear and concise description of what the bug is.

The following code

processed = ""
splits = [1, 2]
lines = ["test", "test", "test"]
processed += "".join(lines[splits[-1]:])

is reformated by black as

processed = ""
splits = [1, 2]
lines = ["test", "test", "test"]
processed += "".join(lines[splits[-1] :])

where a space is added in lines[splits[-1] :].

This breaks flake8

To Reproduce Steps to reproduce the behavior:

Just apply black to a Python file containing what I showed above

Expected behavior A clear and concise description of what you expected to happen.

I expect black to respect E203

Environment (please complete the following information):

• Version: master (959848c)
• OS and Python version: macOS 10.15.4, Python 3.7.6

Does this bug also happen on master? To answer this, you have two options:

Yes, tested on master (959848c)

[hugovk]

This is intentional:

• https://github.com/psf/black/blob/master/README.md#slices

[choucavalier]

I see, thanks!