Remove pvextractor from dependencies

[Carifio24]

This PR removes pvextractor from the dependencies, as it's not used anywhere within the glue-core codebase. The functionality that uses pvextractor all now lives in glue-qt (where it's already a dependency).

The motivation for this is glue-viz/glue-wwt#111. The dependency on pvextractor here means that qtpy is an upstream dependency for glue-wwt, and more generally anything that depends on glue-core.

[neutrinoceros]

Note that qtpy is still an indirect dependency via echo

$ uv pip tree --invert --package qtpy 
qtpy v2.4.1
└── echo v0.8.0
    ├── glue-core v1.21.1
    │   ├── glue-vispy-viewers v1.2.1
    │   │   └── glueviz v1.2.0
    │   └── glueviz v1.2.0
    └── glue-vispy-viewers v1.2.1 (*)
(*) Package tree already displayed

[Carifio24]

@neutrinoceros Yeah, that's true. I maybe should've mentioned it in this PR description but glue-viz/echo#38 is intended to resolve that.

[astrofrog]

This should now be resolved with echo v0.9.0 released (without the qtpy dependency)