Switch from travis-ci to GitHub workflow.

.github/workflows/main.yaml

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+name: Main CI
+
+on: [push]
+
+jobs:
+  #lint:
+  # FIXME
+  test:
+    runs-on: ubuntu-latest
+    #needs: [lint]
+    timeout-minutes: 10
+    strategy:
+      matrix:
+        python-version:
+          #- '3.4'
+          #- '3.5'
+          #- '3.6'
+          - '3.7'
+          - '3.8'
+          - '3.9'
+          - '3.10'
+          - '3.11'
+          - '3.12'
+          - 'pypy3.10'
+        loader: [requests, aiohttp]
+        #exclude:
+        #  - python-version: "3.4"
+        #    loader: aiohttp
+    steps:
+    - uses: actions/checkout@v4
+    - name: Use Python ${{ matrix.python-version }}
+      uses: actions/setup-python@v4
+      with:
+        python-version: ${{ matrix.python-version }}
+        cache: 'pip'
+    - name: Install
+      run: |
+        python -m pip install --upgrade pip
+        pip install -r requirements.txt
+    - name: Fetch test suites
+      run: |
+        git clone --depth 1 https://github.com/w3c/json-ld-api.git _json-ld-api
+        git clone --depth 1 https://github.com/w3c/json-ld-framing.git _json-ld-framing
+        git clone --depth 1 https://github.com/json-ld/normalization.git _normalization
+    - name: Test with Python=${{ matrix.python-version }} Loader=${{ matrix.loader }}
+      run: |
+        python tests/runtests.py ./_json-ld-api/tests -l ${{ matrix.loader }}
+        python tests/runtests.py ./_json-ld-framing/tests -l ${{ matrix.loader }}
+        python tests/runtests.py ./_normalization/tests -l ${{ matrix.loader }}
+      env:
+        LOADER: ${{ matrix.loader }}
+  #coverage:
+  #  needs: [test]
+  # FIXME