May 2018:

pychm has no python3 version. The pull request I submitted for the port is
sitting there, and so is the Debian bug.

https://github.com/dottedmag/pychm/pull/5

Which is why Recoll bundles pychm, enhanced for Python3, for now. The
source repo is here:

https://github.com/medoc92/pychm