Enables setting "usedevelop" per virtualenv. If "[tox]skipsdist" is not explicitly set, it defaults to True if all environments in the current envlist have develop = True.
So, this retains the prior functionality of automatically skipping the sdist step if either "usedevelop" is set for all environments in the "envlist" or the "--develop" option is provided. An explicit setting of "skipsdist" will override this.
The "skipsdist" setting generally tells tox not to build the sdist package, but I'm not sure if that's what you mean. In this case it's not running sdist when requested to only do "setup.py develop" instead, though there may be other use cases that @Monty Taylor had in mind.
I've updated this with a change to set the default for "skipsdist" at config parse time instead.