separate the travis build that makes the docs from the one that runs the tests

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We could use tox for this; another project I work on, Mimic, does it like this: https://github.com/rackerlabs/mimic/blob/master/tox.ini

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As per feedback on #40

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@multani I would like to assign this to you - please accept membership in the Contributors team and then assign it to yourself.

(And please remember NOT to push to the "twisted" repository, that is still only mirrored on github, not developed here…)

[multani]

@glyph What did you have in mind exactly?

Should it completely replace the configuration made in the Travis configuration file? In this case, I don't know if I'll be able to replicate all the details of the current Travis build into Tox, I'm not sure it offers that level of control. (Is that really necessary, though?)

Or should it be a mix between what's currently done in Travis and somehow adding another tox or doc "mode"? Or running Tox to build the documentation each time?

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@multani : I think it can completely replace the "configuration" (by which I think you mean "giant shell script" 😉 ). Did you look at the link to mimic's tox.ini above? It sets up a bunch of tox environments and targets, and then the travis configuration just runs it with TOX_ENV set to each different thing, and we get the desired build matrix.