Add Python 3.10 to GitHub Actions workflow

Python versions must now be quoted in YAML. `3.10` (without quotes) is
interpreted as a float and becomes `3.1`. `"3.10"` will be used instead.

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.8, 3.9]
+        python-version: ["3.8", "3.9", "3.10"]
     env:
       CODECOV_UPLOAD: true
       PIPX_VERSION: "0.16.4"
@@ -59,13 +59,13 @@ jobs:
         run: poetry run pytest --cov-report=xml
       - name: Upload test coverage report to Codecov
         uses: codecov/codecov-action@v2
-        if: env.CODECOV_UPLOAD == 'true' && matrix.python-version == 3.9
+        if: env.CODECOV_UPLOAD == 'true' && matrix.python-version == '3.10'
         with:
           fail_ci_if_error: true
           flags: unit
-      - name: Build Python package with latest Python version and publish to PyPI
+      - name: Build Python package with latest stable Python version and publish to PyPI
         if: >
           env.PYPI_PUBLISH == 'true' &&
-          matrix.python-version == 3.9 &&
+          matrix.python-version == '3.10' &&
           startsWith(github.ref, 'refs/tags/')
         run: poetry publish --build -u __token__ -p ${{ secrets.PYPI_TOKEN }}