Test Lab 3 on Windows, Lab3-compatible packages for demo/roadmap

[bollwyvl]

Follow on to #30 (comment) (thanks again @jtpio):

• puts windows under test with lab 3 🤞
• adds a few more "roadmap" extensions 🚗
  • not really claiming support for them, but mostly to know if one of them causes significant testability issues (e.g. novel unasked-for-stop-the-world-dialogs shakes fist at build recommended)
    • jupyterlab_robotmode
    • jupyterlab_classic
    • voila
    • would consider other suggestions
• still not adding a kernel yet, as they both carry pretty heavy dependency chains
  • would consider for one-off, not-under-CI lockfile, e.g. binder- or docs-only
• validate on binder
  • 
    • don't mind the menu/search box 😊

[bollwyvl]

I think it's looking good...