Fix poetry install script

Closes dry-python#494

References:
- https://python-poetry.org/docs/#installing-with-the-official-installer

.github/workflows/test.yml

@@ -23,8 +23,7 @@ jobs:
 
     - name: Install poetry
       run: |
-        curl -sSL \
-          "https://raw.githubusercontent.com/python-poetry/poetry/master/install-poetry.py" | python
+        curl -sSL https://install.python-poetry.org | python3 -
 
         # Adding `poetry` to `$PATH`:
         echo "$HOME/.poetry/bin" >> $GITHUB_PATH