E203: False positive "whitespace before ':' " on list slice.

I've encountered the problem in the following code:

a = [1, 2, 3, 4, 5]
b = a[1+1 : 2+2]  # E203
c = a[1 + 1 : 2 + 2]  # E203
d = a[1+1:2+2]

However, PEP8 chapter https://www.python.org/dev/peps/pep-0008/#whitespace-in-expressions-and-statements handles this as good style:

However, in a slice the colon acts like a binary operator, and should have equal amounts on either side (treating it as the operator with the lowest priority). In an extended slice, both colons must have the same amount of spacing applied. Exception: when a slice parameter is omitted, the space is omitted.

Yes:

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]
No:

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

Run the three slice examples through pycodestyle and compare the E203 results with the PEP 8 guidance quoted in the issue. Locate the E203 checker and its existing tests in the repository; done means valid slice spacing no longer produces a false positive while invalid spacing remains reported.