add packaging as a dependency

[asottile]

resolves #702

[paugier]

Hum, do we really need to add this dependency just to check if pycodestyle.missing_whitespace_around_operator exists?

#699 was simpler and does not introduce this dependency.

[hhatto]

@asottile @paugier
I mistakenly thought the 'packaging' module was a standard module.
I've made corrections to incorporate pull-request #699 and released version 2.0.4.