Broken walrus support

[canassa]

Hello,

The latest release of pyflakes doesn't seem to support the walrus operator at all:

"pyflakes" failed during execution due to "'FlakesChecker' object has no attribute 'NAMEDEXPR'"

I tried to install the master version but it still doesn't work properly:

models.py:309:66: E231 missing whitespace after ':'
        biocompatible = {m.name: "" for d in data["items"] if (m := d["material"]).biocompatible}

[asottile]

E231 is not pyflakes, that's pycodestyle

#457 fixed the first error (which is already on master)