Exporting fails in blender >2.90

[laochailan]

Steps to reproduce

1, Install and enable iqm addon from the blender-2.80 folder

2. Attempt to export any model as iqm
3. The export fails with the error message

   
   Error: Python: Traceback (most recent call last):
   File "/home/user/.config/blender/2.91/scripts/addons/iqm_export.py", line 1122, in __init__
   bpy.context.scene['iqm_exporter_settings'] = {}
   AttributeError: Writing to ID classes in this context is not allowed: Scene, Scene datablock, error setting Scene.iqm_exporter_settings

location: :-1

This was tested with both blender 2.90 and 2.91.

### Cause
I bisected it to f691b81, which seems plausible in light of the error message.

[Buom01]

The same error also happen on 2.80r3 Using https://raw.githubusercontent.com/lsalzman/iqm/04f540840ac6758140c7fc4afc8e436a72a408fe/blender-2.80/iqm_export.py helped (don't forget to properly remove the previous one)

[nonunknown]

I sucessfully exported using 2.90. What I did was:

• Triangulate the mesh
• Seems that An armature is required in order to export