Add tox configuration

Tox manages test environments for all supported Python versions, as
well as linting and formatting tools. On GitHub, the test and lint
steps are kept as close as possible to the Makefile equivalents.

.github/workflows/ci.yml

@@ -32,11 +32,7 @@ jobs:
       - name: Install linting requirements
         run: poetry install --no-root --with=linting --all-extras
       - name: Execute linters
-        run: |
-          poetry run black --check --diff .
-          poetry run flake8
-          poetry run mypy
-          poetry run pyright
+        run: make lint
 
   test:
     needs:
@@ -59,11 +55,10 @@ jobs:
           cache: poetry
       - name: Install testing requirements
         run: |
-          poetry install --no-root --with=dev --all-extras
+          poetry install --no-root --all-extras
+          poetry run pip install tox-gh-actions
       - name: Execute tests
-        env:
-          PYTHONDEVMODE: '1'
-        run: poetry run pytest
+        run: poetry run tox
 
   test-summary:
     name: Test matrix status