Update version of setuptools

[stsewd]

the 40.x releases has a lot of bugfixes and compatibility changes
to support the latest version of pip, which we install by default now.

Changelog https://setuptools.readthedocs.io/en/latest/history.html#v40-8-0

Closes #5436

[humitos]

I don't have too much experience upgrading this package on RTD, but I think it's safe.

Now, both positive and negative will return the same value. Is that OK?

Besides, since we are modifying the negative value, this will affect all the builds not only the ones that have the feature flag.

[stsewd]

Now, both positive and negative will return the same value. Is that OK?

There isn't a 41.x release. I can just drop the flag and use only one version, but I left it just to be clear that we experiment with that package.

[ericholscher]

Seems fine to me. A bit silly to have both values return the same, but fine for now, especially if we want to use it again in the future.