Works fine for me:
Code:
calibre-debug -c "from calibre.library.comments import comments_to_html; print(comments_to_html.__module__); import sys; print(sys.version_info)"
calibre.library.comments
sys.version_info(major=3, minor=8, micro=0, releaselevel='final', serial=0)
Remember if you are using CALIBRE_DEVELOP_FROM you cannot use the same one for both python 2 and python 3