Update Poetry installs for install-poetry.py

python-poetry/poetry#3706
python-poetry/poetry#3870
python-poetry/poetry#4056

Poetry has a new install script, added in python-poetry/poetry#3706.
The old get-poetry.py install script is not compatible with Python 3.10.
This commit will update the GitHub Actions workflow to use the new
install-poetry.py script, with `POETRY_HOME=/opt/poetry` for consistent
installs independent of user account, and will also update the workflow
to use a virtualenv and run commands with `poetry run` to avoid issues
with `POETRY_VIRTUALENVS_CREATE=false`.

As of Poetry 1.1.7, there may be complications with install-poetry.py
run without venvs (`POETRY_VIRTUALENVS_CREATE=false`). An error is seen:

```text
OSError

Could not find a suitable TLS CA certificate bundle, invalid path:
/opt/poetry/venv/lib/python3.9/site-packages/certifi/cacert.pem

at /opt/poetry/venv/lib/python3.9/site-packages/requests/adapters.py:227
in cert_verify
```

Poetry may be incorrectly attempting to read from its virtualenv if it's
not respecting `POETRY_VIRTUALENVS_CREATE` (python-poetry/poetry#3870).
Downstream steps also do not respect `POETRY_VIRTUALENVS_CREATE`, so the
application does not run. To avoid these issues, `poetry run` can be
prepended to commands to prompt Poetry to use its virtualenv.

Additionally, Poetry errors out with a `JSONDecodeError` when attempting
to install packages with Python 3.10 (python-poetry/poetry#4210). Python
3.10 support will be postponed until Poetry is compatible.

.github/workflows/ci.yml

@@ -16,7 +16,7 @@ jobs:
         python-version: [3.8, 3.9]
     env:
       CODECOV_UPLOAD: true
-      POETRY_VIRTUALENVS_CREATE: false
+      POETRY_HOME: /opt/poetry
       PYPI_PUBLISH: true
     steps:
       - uses: actions/checkout@v2
@@ -39,16 +39,16 @@ jobs:
           restore-keys: ${{ runner.os }}-pre-commit-
       - name: Install Poetry
         run: |
-          curl -fsS -o get-poetry.py \
-            https://raw.githubusercontent.com/python-poetry/poetry/HEAD/get-poetry.py
-          python get-poetry.py -y
-          echo "$HOME/.poetry/bin" >> $GITHUB_PATH
+          curl -fsS -o install-poetry.py \
+            https://raw.githubusercontent.com/python-poetry/poetry/HEAD/install-poetry.py
+          python install-poetry.py -y
+          echo "$POETRY_HOME/bin" >> $GITHUB_PATH
       - name: Install dependencies
         run: poetry install --no-interaction
       - name: Run pre-commit hooks
-        run: pre-commit run --all-files
+        run: poetry run pre-commit run --all-files
       - name: Run unit tests
-        run: pytest --cov-report=xml
+        run: poetry run pytest --cov-report=xml
       - name: Upload test coverage report to Codecov
         uses: codecov/codecov-action@v1
         if: env.CODECOV_UPLOAD == 'true'