Testing with pip cache purge prior to install pip packages

[kronos30]

The following is a test to see if clearing the pip cache prevents the weird situation where pip cannot install a package based on requirements.txt. There are instances when a package on the build farm will fail due to venv not being able to pip install the package, even though it exists in pypi.

In #1 discusses about how pip will often use locally built cached wheels first and how by default pip will use local caching first. Perhaps this could be where there is an issue.

[1] https://pip.pypa.io/en/stable/topics/caching/