Report incorrect assigning of dictionary entries with a colon (`:`)

[dannysepler]

Consider the following code

dct = {
    'a': 1,
}
dct['b']: 2
print(dct)  # {'a': 1}

This is actually valid python syntax, because it is typing the 'b' entry as a 2 type. But we can assume the user meant to assign an element rather than type it (especially if there's no equal on this line).

Maybe this is better for flake8-bugbear instead, happy to open an issue there.

[asottile]

probably better for bugbear yeah -- this is a (imo) wart of the typing system allow you to annotate any expression

probably needs to allow x: Ann, and x.y: Ann and forbid everything else? there was some discussion of this on typing-sig at one point but I can't find it

[asottile]

linked issue is above ^^ thanks!