fix(calendar): Missing python-dateutil in package.nix

[lazakoa]

Pull Request

Motivation

This fixes an issue with calendar-events.py imports dateutil.relativedelta for monthly/yearly recurrence, but python-dateutil isn't in the Python environment, so recurring events silently fail and skip remaining events in that calendar. Tested on NixOS and niri.

Type of Change

Mark the relevant option with an "x".

• Bug fix
• New feature
• Breaking change
• Refactoring

Testing

Describe how you tested your changes and mark the relevant items.

• Tested on niri
• Tested on Hyprland
• Tested on sway
• Tested with different bar positions and density settings
• Tested at different interface scaling values
• Tested with multiple monitors (if applicable)

Screenshots / Videos

None.

Checklist

• Code follows project style guidelines
• Self-reviewed my code
• No new warnings or errors
• Documentation or comments updated (if relevant)

Additional Notes

None.