chore: add license check action (#355)

Signed-off-by: Tao Feng <fengtao04@gmail.com>

.github/workflows/license.yml

+name: license
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up Golang
+      uses: actions/setup-go@v2
+    - name: Install addlicense
+      run: |
+        export PATH=${PATH}:`go env GOPATH`/bin
+        go get -v -u github.com/google/addlicense
+    - name: Check license
+      run: |
+        export PATH=${PATH}:`go env GOPATH`/bin
+        addlicense -check -l mit -c "Amundsen" $(find $PWD -type f -name '*.py')
\ No newline at end of file

databuilder/extractor/dashboard/tableau/__init__.py

+# Copyright Contributors to the Amundsen project.
+# SPDX-License-Identifier: Apache-2.0

databuilder/extractor/dashboard/tableau/tableau_dashboard_constants.py

+# Copyright Contributors to the Amundsen project.
+# SPDX-License-Identifier: Apache-2.0
+
 API_VERSION = 'api_version'
 API_BASE_URL = 'api_base_url'
 TABLEAU_BASE_URL = 'tableau_base_url'

databuilder/extractor/dashboard/tableau/tableau_dashboard_extractor.py

+# Copyright Contributors to the Amundsen project.
+# SPDX-License-Identifier: Apache-2.0
+
 import logging
 from typing import Any, Dict, Iterator, List
 

databuilder/extractor/dashboard/tableau/tableau_dashboard_last_modified_extractor.py

+# Copyright Contributors to the Amundsen project.
+# SPDX-License-Identifier: Apache-2.0
+
 import logging
 from typing import Any, Dict, Iterator, List
 

databuilder/extractor/dashboard/tableau/tableau_dashboard_query_extractor.py

+# Copyright Contributors to the Amundsen project.
+# SPDX-License-Identifier: Apache-2.0
+
 import logging
 from typing import Any, Dict, Iterator
 

databuilder/extractor/dashboard/tableau/tableau_dashboard_table_extractor.py

+# Copyright Contributors to the Amundsen project.
+# SPDX-License-Identifier: Apache-2.0
+
 import logging
 from typing import Any, Dict, Iterator
 

databuilder/extractor/dashboard/tableau/tableau_dashboard_utils.py

+# Copyright Contributors to the Amundsen project.
+# SPDX-License-Identifier: Apache-2.0
+
 import json
 import requests
 import re

databuilder/extractor/dashboard/tableau/tableau_external_table_extractor.py

+# Copyright Contributors to the Amundsen project.
+# SPDX-License-Identifier: Apache-2.0
+
 import logging
 from typing import Any, Dict, Iterator