Updated PythonAnywhere virtualenv instructions

[gpjt]

I appreciate that this is an old project and perhaps isn't being updated :-)

However, on tech support for PythonAnywhere we've had an uptick in people using LLMs to guide them through the Flask deployment process, and one problem they keep tripping over is that they're using activate_this in their WSGI files, which was the pattern we recommended back in the day, and is documented in lots of places, including here. Unfortunately now it causes more problems than it solves.

We've been scouring the Internet for mentions to see if we can change this so that the next training runs know the up-to-date way to do this, and would really appreciate it if this could be merged.