wxGUI/iclass: fix showing scatter plots

[tmszi]

Describe the bug
Supervised Classification Tool does not show scatter plots and prints error message.

To Reproduce
Steps to reproduce the behavior:

1. Launch Supervised Classification Tool wxGUI g.gui.iclass
2. On the left Plots pane combobox widget choose Scatter plots item
3. On the Scatter plots toolbar activate Add scatter plot tool
4. On the Select imagery group dialog from the name of imagery group listbox widget items choose lsat7_2000
5. Hit Create/edit group... button widget
6. On the Create or edit image group dialog, type into Select existing subgroup or enter name of new subgroup combobox widget new subgroup name e.g. test and select some maps from the list to be part of the new cretaed subgroup test (lsat7_2000_10, lsat7_2000_20, lsat7_2000_30, lsat7_2000_40) and hit OK button
7. Hit OK button on the Create or edit image group dialog
8. On the next Add scatter plots dialog choose from the x axis combobox widget items lsat7_2000_10 raster map, and from the y axis combobox widget items choose lsat7_2000_20 raster map and hit Add button
9. Repeat point 9. but with another maps, from the x axis combobox widget items choose lsat7_2000_30 raster map, and from the y axis combobox widget items choose lsat7_2000_40 raster map and hit Add button
10. On the Add scatter plots dialog hit OK button
11. See error

First error:

Exception in thread Thread-4:
Traceback (most recent call last):
  File "/usr/lib/python3.10/threading.py", line 1016, in _bootstrap_inner
    self.run()
  File "/usr/lib64/grass84/gui/wxpython/core/gthread.py", line 151, in __run
    self.__run_backup()
  File "/usr/lib64/grass84/gui/wxpython/core/gthread.py", line 114, in run
    ret = vars()["callable"](*args, **kwds)
  File "/usr/lib64/grass84/gui/wxpython/iscatt/controllers.py", line 604, in _renderscattplts
    scatt["scatt"].Plot(
  File "/usr/lib64/grass84/gui/wxpython/iscatt/plots.py", line 234, in Plot
    img = imshow(
  File "/usr/lib64/grass84/gui/wxpython/iscatt/plots.py", line 1021, in imshow
    if not axes._hold:
AttributeError: 'Axes' object has no attribute '_hold'

Second error:

Exception in thread Thread-4:
Traceback (most recent call last):
  File "/usr/lib/python3.10/threading.py", line 1016, in _bootstrap_inner
    self.run()
  File "/usr/lib64/grass84/gui/wxpython/core/gthread.py", line 151, in __run
    self.__run_backup()
  File "/usr/lib64/grass84/gui/wxpython/core/gthread.py", line 114, in run
    ret = vars()["callable"](*args, **kwds)
  File "/usr/lib64/grass84/gui/wxpython/iscatt/controllers.py", line 604, in _renderscattplts
    scatt["scatt"].Plot(
  File "/usr/lib64/grass84/gui/wxpython/iscatt/plots.py", line 234, in Plot
    img = imshow(
  File "/usr/lib64/grass84/gui/wxpython/iscatt/plots.py", line 1070, in imshow
    axes.images.append(im)
AttributeError: 'ArtistList' object has no attribute 'append'

Expected behavior
Supervised Classification Tool should show scatter plots without error message.

Screenshots

Expected behavior:

System description:

• Operating System: all
• GRASS GIS version: all

GRASS nc_spm_08_grass7/landsat:~ > python3 -c "import sys, matplotlib, wx; print(sys.version); print(wx.version()); print(f'{matplotlib.__version__} matplotlib')"
3.10.13 (main, Sep 16 2023, 22:24:59) [GCC 12.3.1 20230526]
4.2.0 gtk3 (phoenix) wxWidgets 3.2.2.1
3.7.2 matplotlib

Additional context

Matplotlib v3.0 API changes:

matplotlib.axes.Axes.hold doc:

https://matplotlib.org/2.2.5/api/_as_gen/matplotlib.axes.Axes.hold.html