How to fix Python ModuleNotFoundError: No module named 'pillow_avif'

If you see an error message such as

Traceback (most recent call last):
  File "/home/uli/Nextcloud/aviftest/./MisterAvif.py", line 11, in <module>
    import pillow_avif
ModuleNotFoundError: No module named 'pillow_avif'

you need to install the pillow-avif-plugin Python module. This is a separate module from the main Pillow module.

To install it, run

pip install pillow-avif-plugin