wxGUI/vdidit: fix opening Vector Digitizer settings dialog while editing vector map

[tmszi]

Describe the bug
Opening vector digitizer settings dialog while editing vector map fails.

To Reproduce
Steps to reproduce the behavior:

1. Launch Vector Digitizer e.g. g.gui.vdigit geology
2. From the Vector Digitizer window main toolbar activate Settings tool
3. See error

Traceback (most recent call last):
  File "/usr/lib64/grass84/gui/wxpython/vdigit/toolbars.py", line 698, in OnSettings
    self.settingsDialog = VDigitSettingsDialog(
  File "/usr/lib64/grass84/gui/wxpython/vdigit/preferences.py", line 48, in __init__
    self._createAttributesPage(notebook)
  File "/usr/lib64/grass84/gui/wxpython/vdigit/preferences.py", line 638, in _createAttributesPage
    column.InsertColumns(
  File "/usr/lib64/grass84/gui/wxpython/gui_core/gselect.py", line 1176, in InsertColumns
    for key, value in columnchoices.item():
AttributeError: 'dict' object has no attribute 'item'. Did you mean: 'items'?

Expected behavior
Opening vector digitizer settings dialog while editing vector map should work.

System description:

• Operating System: all
• GRASS GIS version: 8.4.0dev

GRASS nc_basic_spm_grass7/PERMANENT:~ > python3 -c "import sys, wx; print(sys.version); print(wx.version())"
3.10.13 (main, Aug 25 2023, 20:13:27) [GCC 12.3.1 20230526]
4.2.0 gtk3 (phoenix) wxWidgets 3.2.2.1

[tmszi]

Backport to 8.3, 8.2 is no needed.