Upstream Freedoom maintainer here. We have dropped support for Python 2 entirely in the VCS HEAD (by means of replacing all shebangs from invoking "python" to "python3"), but 0.11.3 should also be completely compatible with Python 3. Possible workarounds for the Debian package might either be to use a python venv, run an "alias python=python3" before building, or patch all the invocation lines for python3.
A similar issue arises too with the use of AsciiDoc, whose reference implementation only has official releases that have a hard depend on python2. I think I will add an ASCIIDOC variable to the Makefiles to control the implementation used, so it could be switched to asciidoctor for example; this should be a trivial patch to backport. Unfortunately there is no plan to make a 0.11.4 release, but we are trying to get a 0.12.0 as soon as we can :)