Add constraints to python2 plugin

[SamYaple]

LP:#1603110

There is currently no support for constraints[1] files with the python2 plugin. Some projects make heavy use of constraints. OpenStack is one such project

[1] https://pip.pypa.io/en/stable/user_guide/#constraints-files

[snappy-m-o]

Can one of the admins verify this patch?

[snappy-m-o]

Can one of the admins verify this patch?

[SamYaple]

this PR is waiting for other PRs which improve the test coverage of the python2 module, at that point this will be rebased

[sergiusens]

unit tests failed here

[sergiusens]

but good work so far :-)

[sergiusens]

snapcraft/plugins/python2.py:126:1: C901 'Python2Plugin._pip' is too complex (10)

./runtests.sh static to reproduce locally

[javacruft]

It would be nice if the python3 plugin could support constraints as well - it has general applicability across pip installations.

We might want to rework the python plugins a bit to have a common base - I can see alot of common code.

[SamYaple]

@javacruft thats next, want to get this working and approved before implementing py3 patch

refactoring isn't the worst idea. ill look into it. just trying to get these plugins usable for my needs

[SamYaple]

@sergiusens running locally doesnt produce that error for complexity, even after rebase onto master. I probably havent pushed something up so ill check on it.

[sergiusens]

ok to test

[sergiusens]

Thanks for this