Hi,

Since pip doesn't have a great dependency resolver (see pypa/pip#988) it will download the latest version of the dependencies of the first package passed to it. For example,

pip install tox pytest

This will install py>=1.4.17 as depended on by tox, which will happily get the most recent py package at version 1.5. However, we cannot run pytest now since pytest depends on py<1.5

I think you can make the argument that the blame doesn't fall within pytest itself, but I think there's going to be a lot of requirements.txt out there which don't list pytest first, and may run into this issue.