Fix for pip 8.1.2

Hi! I released devpi-common 2.0.9 and devpi-server 3.1.1 to fix https://github.com/pypa/pip/issues/3666 For devpi-server 2.6.1 the update to devpi-common 2.0.9 is enough. I only released devpi-server 3.1.1 because of a change in the import code in 3.1.0 which was also affected by this. I tested devpi-common 2.0.9 with devpi-server/web 2.6.x and devpi-server/web 3.1.x. Regards, Florian Schulze