setup CI workflow with tox

.github/workflows/ci.yml

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+name: CI
+
+on:
+  pull_request:
+    paths:
+      - carling/**
+      - .github/workflows/ci.yml
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python: [3.7, 3.8, 3.9]
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Set up Python
+        uses: actions/setup-python@v2
+        with:
+          python-version: ${{ matrix.python }}
+      - name: Install dependencies
+        run: |
+          python -m pip install poetry tox
+      - name: Test with tox
+        run: poetry run tox

tox.ini

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+[tox]
+envlist =
+    py{37,38,39}
+    black
+    flake8
+    isort
+isolated_build = true
+skipsdist = true
+skip_missing_interpreters = true
+
+[testenv]
+skip_install = true
+whitelist_externals = poetry
+commands =
+    poetry install
+    poetry run pytest -vv tests
+
+[testenv:black]
+deps = black
+commands = poetry run black --check .
+
+[testenv:flake8]
+deps = flake8
+commands = poetry run flake8 .
+
+[testenv:isort]
+deps = isort
+commands = poetry run isort --check .