I should add that you probably rolled back the upgrade and removed the keg (i.e., `/usr/local/Cellar/python/3.7.0`) entirely, rendering the venv stranded. I believe if you simply switch to a previous 3.6.5 keg with `brew switch python 3.6.5` and leave the 3.7.0 keg alone, buku should still work.
Sorry, didn't see the previous ping.
The formula does not include "its own python3". When it is installed, Homebrew creates a venv with Homebrew's python3, then installs the dependencies inside. The dependencies are completely contained, isolated and reproducible (and hence guaranteed to work reproducibly), which is not possible with a generic shebang.
It sounds to me like you rolled back the Homebrew python37 upgrade on your own (which is not recommended; use, say, pyenv instead) and did not `brew reinstall buku` afterwards.