text: Title: Use alternative mechanism(s) to upgrade shillelagh
  Body: `shillelagh.functions.upgrade`  uses `pip` to upgrade the module.  This is a hidden dependency that is not automatically installed.
  
  If using a different package manager such as `uv` I have to add `pip` as a requirement for my project, which doesn't seem right.
  
  Either remove the upgrade function all together, or use a different mechanism to upgrade within the tool.