Create Python virtual environment as part of build

[alexreinking]

We should create and configure Python venvs that read the in-repo python_bindings/requirements.txt as part of the normal build flow. This will make setting up new workers more robust and not add much in execution time because pip / venv does sane binary caching. It will also let us upgrade our existing dependencies and add new ones easily.

[alexreinking]

Bumping this in importance