Issue #2731: Constraints files.

[rbtcollins]

This adds constraints files. Like requirements files constraints files
control what version of a package is installed, but unlike
requirements files this doesn't itself choose to install the package.
This allows things that aren't explicitly desired to be constrained if
and only if they are installed.

[rbtcollins]

Failure on py27 is unrelated to patch - Download error on https://pypi.python.org/simple/INITools/: [Errno 110] Connection timed out -- Some packages may not be found!

[qwcode]

neat, +1

[dstufft]

This looks OK to me, my only thing is not a "thing to fix" exactly, but more of thinking if we want to claim the -c short option or not. This isn't something I've ever wanted and I don't have a good gauge of how many people are going to use it. If this is somewhat of a niche case then maybe we should only use the long form? Alternatively, maybe it's got a lot of use cases that I'm just not thinking of and a lot of folks are going to use it :)

[rbtcollins]

Ultimately thats your call; but unless you have other things clamouring for -c, I'd like to use it ;).

As for usage - I can fairly easily see this being more popular than requirements files, for orgs with multiple repositories and non-homogeneous installs. E.g. three services, each in their own container, use constraints to keep everything aligned.

[qwcode]

yea, I'm wary of claiming the -c, but I don't feel that strongly about it.

three services, each in their own container, use constraints to keep everything aligned.

sorry, I don't follow how contraints would be "more popular"? yes, I can see using them to enforce certain general patches across your service environments, but still these services could still want frozen requirements files for dependencies specific to themselves?

[rbtcollins]

I can imagine folk using constraints files in combination with requirements files, or for one-service-per-venv situations, instead of: particularly when you want to keep things in sync rather than each deployed thing being different.

[dstufft]

Ok I'm going to merge that. I feel similarly to @qwcode, I'm wary of claiming -c but I don't feel strongly about it. Given I can't think of anything else to use for -c and this came first, we'll go ahead and give it that.