v0.3: Support for Python3.9+ and Qt6

What's Changed

• Move to Qt6 to support Python 3.9 to 3.12 by @NickGeek in #12

⚠️ Breaking changes

Users with Python 3.6-3.8 or Qt5 should keep using v0.2, because this new version requires PySide6, which is not compatible with Qt5.

Full Changelog: v0.2...v0.3