Update to PyQT6

Hello, I'd like to ask if there are plans to use the newer PyQT6 instead of PyQT5, the current open source version has support for it already (see https://github.com/schrodinger/pymol-open-source/blob/9c48a9917d0f9a23bc5471ba105f467aaa017ed9/modules/pymol/Qt/__init__.py#L51).

## System information

```
>flatpak info org.pymol.PyMOL
Python Molecular Graphics - Molecular visualization and raytracing

ID: org.pymol.PyMOL
Ref: app/org.pymol.PyMOL/x86_64/stable
Arch: x86_64
Branch: stable
Version: 3.1.0
License: Python-2.0
Origin: flathub
Collection: org.flathub.Stable
Installation: system
Installed: 416,0 MB
Runtime: org.kde.Platform/x86_64/5.15-24.08
Sdk: org.kde.Sdk/x86_64/5.15-24.08

Commit: b57042129982ee6682056f4cf194796e99a64aec087feeccd60f22740bc99fa4
Parent: 6b33e11de98f9e37174e95979ad8d6dfa599b202967927c2673c185cadb09b26
Subject: Merge pull request #143 from flathub/limit-flathubbot (edb0c0c2e72f)
Date: 2025-12-24 22:18:41 +0000
```