...
cmdargs: ['/home/travis/build/prometheus/client_python/.tox/py26/bin/pip', 'install', 'coverage', 'pytest', 'unittest2']
...

Collecting py>=1.5.0 (from pytest)
  Could not find a version that satisfies the requirement py>=1.5.0 (from pytest) (from versions: 0.9.1, 0.9.2, 1.0.0, 1.0.1, 1.0.2, 1.1.0, 1.1.1, 1.2.0, 1.2.1, 1.3.0, 1.3.1, 1.3.2, 1.3.3, 1.3.4, 1.4.0, 1.4.1, 1.4.2, 1.4.3, 1.4.4, 1.4.5, 1.4.6, 1.4.7.dev3, 1.4.7, 1.4.8, 1.4.9, 1.4.10, 1.4.11, 1.4.12, 1.4.13, 1.4.14, 1.4.15, 1.4.16, 1.4.17, 1.4.18, 1.4.19, 1.4.20, 1.4.21, 1.4.22, 1.4.23, 1.4.24, 1.4.25, 1.4.26, 1.4.27, 1.4.28, 1.4.29, 1.4.30, 1.4.31, 1.4.32, 1.4.33, 1.4.34)
No matching distribution found for py>=1.5.0 (from pytest)