I think something changed in the way pip handles dependencies or installs packages... I spent quite some time yesterday trying to fix GC3Pie/Py2.6 tests (which fail for a similar reason).

GC3Pie tests fail because pycparser (which is a dependency of PyNaCl, which in turns is required by Paramiko, which GC3Pie uses) uses 2.7-only syntax in version 2.19. So if you depend on pycparser==2.18 it should work, right? Well, it turns out that pip does install pycparser==2.18 according to the Travis CI logs, but then ignores it and still tries to install the latest pycparser when installing PyNaCl... even if one lists PyNaCl==1.0.1 (last version to support Py2.6) in the dependencies!

I am really puzzled.

@riccardomurri Which version of pip are you using in that context? We're not seeing that issue with the pip 9.0.3 (which is the latest that supports Python 2.6, it seems, since that was the version available after pip install --upgrade pip) being used by Travis in our tests (see https://travis-ci.org/easybuilders/easybuild-framework/jobs/460323743).
After pip install 'pycparser<2.19' and pip install 'python-daemon<2.0' now, pip install GC3Pie seems to be working just fine...

After pip install 'pycparser<2.19' and pip install 'python-daemon<2.0' now, pip install GC3Pie seems to be working just fine...

But you do this in Travis' script before installing GC3Pie, you do not do it from the setup.py script, right?

@riccardomurri That's right, and then pip picks up on those pre-installed packages when installing GC3Pie since they fit the requirements, see:

Requirement already satisfied: python-daemon in /home/travis/virtualenv/python2.6.9/lib/python2.6/site-packages (from GC3Pie)
...
Requirement already satisfied: pycparser in /home/travis/virtualenv/python2.6.9/lib/python2.6/site-packages (from cffi>=1.1->bcrypt>=3.1.3->paramiko->GC3Pie)