pip plugin origin versioning

This merge request:

• Implements version checking on plugins loaded from the pip origin, as was discussed in this email.

  This is done without any literal implementation as @cs-shadow found it already works as is, only a hand full of error checking was added in this regard.

• Adds some support for testing the codepaths of loading plugins from pip plugin origins which was previously lacking (we now do this by committing a small python package directory structure to our git repository and have tox install it for us in its venv).

• Enhances documentation around both local and pip plugin origins.