CI: Switch to GitHub Actions

Because Travis CI is dead.

.github/workflows/ci.yaml

@@ -0,0 +1,52 @@
+---
+
+name: CI
+
+on:  # yamllint disable-line rule:truthy
+  push:
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  lint:
+    name: Linters
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Set up Python
+        uses: actions/setup-python@v2
+        with:
+          python-version: 3.9
+      - run: python -m pip install flake8 flake8-import-order doc8 sphinx
+      - run: python -m pip install .
+      - run: flake8 .
+      - run: doc8 $(git ls-files '*.rst')
+      - run: yamllint --strict $(git ls-files '*.yaml' '*.yml')
+      - run: python setup.py build_sphinx
+
+  test:
+    name: Tests
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version:
+          - 3.5
+          - 3.6
+          - 3.7
+          - 3.8
+          - 3.9
+          - nightly
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Set up Python ${{ matrix.pyver }}
+        uses: actions/setup-python@v2
+        with:
+          python-version: ${{ matrix.pyver }}
+      - run: python -m pip install coveralls
+      - run: python -m pip install .
+      - run: coverage run --source=yamllint -m unittest discover
+      - name: Coveralls
+        uses: AndreMiras/coveralls-python-action@develop