pip uses now deprecated pytoml

[htgoebel]

• Pip version: 18.1

pip vendors pytoml, which has been deprecated 2019-01-01:

The pytoml project is no longer being actively maintained. Consider using the
toml package instead.

[pradyunsg]

Thanks for filing this @htgoebel!

#6120 covers this indirectly but I'll keep this issue open. :)