Allow setting env vars using Python expressions

[jayvdb]

I would like to use Python expressions for the values, as a way to implement

OS_NAME=$(python -c "import os; print(os.name)")

[jsmedmar]

@jayvdb, it seems the value you want to use might be an environment variable. See the new functionality that was just merged (last section).

[gaborbernat]

Development moved to https://github.com/pytest-dev/pytest-env