E226: pycodestyle incorrectly warns about missing whitespace around operators

Given the following example.py,

my_list = [1, 2, 3]
a = 2+2
b = my_list[1:1+1]

pycodestyle incorrectly warns about the missing whitespace for the + operator inside the list slice:

» pycodestyle --select E226 ./example.py
./example.py:2:6: E226 missing whitespace around arithmetic operator  # OK
./example.py:3:16: E226 missing whitespace around arithmetic operator  # incorrect

The first warning (line 2) is correct (line 2 should read a = 2 + 2); the second warning (line 3) is incorrect according to PEP-8:

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]

Edit: okay, I'm not so sure I've got it 100%. The style I think is PEP-8 doesn't really seem to get listed anywhere, but even if I add spaces around the : (which is listed), I still get lints.

Reproduce the report with the example.py snippet and pycodestyle --select E226. Start by locating pycodestyle's E226 implementation and checking how it handles operators inside list slices. Done means ordinary unspaced arithmetic such as 2+2 still warns, while the reported + inside a slice does not.