hi
i tryied to build app in python3 only environment (fresh ubuntu 18.04)
i changed relevant parts in debian/control file
and run build dpkg-buildpackage -us -uc -b
i get this error:
Can't exec "pyversions": No such file or directory at /usr/share/perl5/Debian/Debhelper/Buildsystem/python_distutils.pm line 121.

so after googling, i found solution
file debian/rules file was needed to edit to void error above
line dh $@ --with python3
change to
line dh $@ --with python3 --buildsystem=pybuild

after those updates i was able to build with no errors
so i am posting git diff of this changes if someone will be interested about it

build-with-python3-only-environment.diff.txt