fix poetry docs

[rmorshea]

Poetry docs are broken:

2023-06-03T22:12:30.486911+00:00 heroku[web.1]: Starting process with command `/bin/sh -c pipx\ run\ poetry\ run\ python\ main.py`
2023-06-03T22:12:32.452922+00:00 app[web.1]: creating virtual environment...
2023-06-03T22:12:32.493175+00:00 app[web.1]: creating shared libraries...
2023-06-03T22:12:36.152850+00:00 app[web.1]: upgrading shared libraries...
2023-06-03T22:12:44.947651+00:00 app[web.1]: installing poetry...
2023-06-03T22:12:56.226991+00:00 app[web.1]: Creating virtualenv docs-DoOuTdRN-py3.9 in /app/docs/.cache/pypoetry/virtualenvs
2023-06-03T22:12:57.064521+00:00 app[web.1]: Traceback (most recent call last):
2023-06-03T22:12:57.064523+00:00 app[web.1]:   File "/app/docs/main.py", line 3, in <module>
2023-06-03T22:12:57.064561+00:00 app[web.1]:     from docs_app import dev, prod
2023-06-03T22:12:57.064561+00:00 app[web.1]:   File "/app/docs/docs_app/dev.py", line 7, in <module>
2023-06-03T22:12:57.064598+00:00 app[web.1]:     from sphinx_autobuild.cli import (
2023-06-03T22:12:57.064604+00:00 app[web.1]: ModuleNotFoundError: No module named 'sphinx_autobuild'

Somehow the venv created by Poetry via pipx was not preserved.