Allow PATH modifications via prependpath and appendpath.

[aklajnert]

Closes #1423

Two changes here:

• Warn if a user attempts to change PATH via setenv,
• Allow extending PATH via prependenv and appendenv variables.

[gaborbernat]

Rather than adding two new settings why don't we just handle PATH instead?

[aklajnert]

Rather than adding two new settings why don't we just handle PATH instead?

You mean, should we allow to change PATH via the setenv? This is potentially breaking change. I imagine that some people might have it now in their tox.ini files, without realizing that it doesn't do anything. I guess that after an update, tox might not even start due to the wrong PATH setting.

[gaborbernat]

I'd consider those people to have a bug in their config so I don't mind breaking those users 👍

[aklajnert]

@gaborbernat Should I implement any safety mechanism to ensure that the user won't shoot himself in the foot? E.g. a warning if the original PATH is not part of the new one?

[gaborbernat]

I would not warn here, IMHO you should not care about host PATH post interpreter discovery.

[aklajnert]

OK, I'll create a fresh PR as I'll have to revert most of the changes anyway.