Slicing with omitted operand is not formatted according to PEP 8

[terrelln]

Black suggests putting a space before the slice operator when the LHS has an operator but the RHS is omitted, which looks like it doesn't comply with PEP-8 (emphasis mine):

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.

Looks like it was introduced by PR #178.

Python version: 3.6
Black version: black, version 18.4a5
Does also happen on master: yes

$ cat test.py
x = "hello"
x[0 + -1:]
$ black --diff test.py
--- test.py  (original)
+++ test.py  (formatted)
@@ -1,3 +1,3 @@
 x = "hello"
-x[0 + -1:]
+x[0 + -1 :]

reformatted test.py

[terrelln]

I misunderstood the PEP 8 wording. The space is omitted on the RHS in the example, which is what the exception was referring to.

Seems a bit strange, but I'll get used to it.