Flake8 does not follow PEP 8 in slices

[levsa]

Example from PEP 8 (see "However, in a slice..."), partially run through black:

File flake_range.py:

ham = list(range(100))
lower = 3
upper = 8
offset = 1
step = 1
x = 4


def upper_fn(y: int) -> int:
    pass


step_fn = upper_fn

# Correct (after black):
ham[1:9], ham[1:9:3], ham[:9:3], ham[1::3], ham[1:9:]
ham[lower:upper], ham[lower:upper:], ham[lower::step]
ham[lower + offset : upper + offset]
ham[: upper_fn(x) : step_fn(x)], ham[:: step_fn(x)]
ham[lower + offset : upper + offset]

# Wrong:
ham[lower + offset:upper + offset]
ham[1: 9], ham[1 :9], ham[1:9 :3]
ham[lower : : upper]
ham[ : upper]

# Wrong (after black):
ham[lower + offset : upper + offset]
ham[1:9], ham[1:9], ham[1:9:3]
ham[lower::upper]
ham[:upper]

Expected result:

/private/tmp/flake_range.py:24:7: E203 whitespace before ':'
/private/tmp/flake_range.py:24:17: E203 whitespace before ':'
/private/tmp/flake_range.py:24:30: E203 whitespace before ':'
/private/tmp/flake_range.py:25:10: E203 whitespace before ':'
/private/tmp/flake_range.py:25:12: E203 whitespace before ':'
/private/tmp/flake_range.py:26:5: E201 whitespace after '['

Actual result:

/private/tmp/flake_range.py:18:19: E203 whitespace before ':'
/private/tmp/flake_range.py:19:18: E203 whitespace before ':'
/private/tmp/flake_range.py:20:19: E203 whitespace before ':'
/private/tmp/flake_range.py:24:17: E203 whitespace before ':'
/private/tmp/flake_range.py:24:30: E203 whitespace before ':'
/private/tmp/flake_range.py:25:10: E203 whitespace before ':'
/private/tmp/flake_range.py:25:12: E203 whitespace before ':'
/private/tmp/flake_range.py:26:5: E201 whitespace after '['
/private/tmp/flake_range.py:29:19: E203 whitespace before ':'

[asottile]

please read the top of the issue template

[simevo]

the issue is tracked in pycodestyle here: PyCQA/pycodestyle#373