`poetry install` installs `dev` dependencies by default

When installing with poetry, `dev` dependencies are installed by default.

Test:
```bash
python -c "import mkdocs; print('mkdocs available')"  # Produces module not found
git clone https://github.com/ppsp-team/HyPyP
cd HyPyP
poetry install
poetry run python -c "import mkdocs; print('mkdocs available')" # Prints "mkdocs available"
```

`mkdocs` is one of the `dev` dependencies, shouldn't be there.