Add GitHub Action to copy labels from issues to PRs. Closes #4176. (#4177)

.github/workflows/label_transfer.yml

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+
+name: Copy labels from issue to pull request
+
+on:
+  pull_request_target:
+    types: [opened, edited]
+
+jobs:
+  transfer_tags:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up Python 3.9
+      uses: actions/setup-python@v2
+      with:
+        python-version: 3.9
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip setuptools
+        pip install -r .github/workflows/label_transfer/requirements.txt
+        pip freeze
+    - name: Run Python label transfer script
+      run: python .github/workflows/label_transfer/script.py ${{ github.event.number }}
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        GITHUB_REPO: ${{ github.repository }}

.github/workflows/label_transfer/requirements.txt

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+requests
+lxml

.github/workflows/label_transfer/script.py

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+import requests
+import os
+from lxml import html
+import sys
+import json
+
+# Config
+
+# list of labels that should not be transferred to PRs
+do_not_transfer = [
+    'good first issue',
+    'help wanted',
+    'duplicate',
+    'invalid',
+    'question',
+    'to be reviewed',
+]
+
+# Internal
+
+repo = os.environ.get('GITHUB_REPO')
+github_token = os.environ.get('GITHUB_TOKEN')
+
+headers = {
+    'Accept': 'application/vnd.github.v3+json',
+}
+
+if github_token:
+    headers['Authorization'] = 'Bearer '+github_token
+
+def get_linked_issues(pr_number):
+    """
+    Given a PR number, extract all the linked issue numbers from it.
+    Sadly this is not supported by the API yet, so we just scrape the web UI.
+    """
+    url = f'https://github.com/{repo}/pull/{pr_number}'
+    page = requests.get(url)
+    page.raise_for_status()
+    parsed = html.document_fromstring(page.text)
+    matches = parsed.xpath('//form/div[@class="css-truncate my-1"]/a')
+    for match in matches:
+        yield int(match.attrib['href'].split('/')[-1])
+
+def get_issue_labels(issue_number):
+    """
+    Returns all the labels in a given issue / PR
+    """
+    url = f'https://api.github.com/repos/{repo}/issues/{issue_number}/labels'
+    response = requests.get(url, headers=headers)
+    response.raise_for_status()
+    return [ tag['name'] for tag in response.json() ]
+
+def transfer_issue_labels(pr_number):
+    """
+    Transfers labels from all the linked issues to the PR
+    """
+    linked_issues = get_linked_issues(pr_number)
+    all_labels = [ label for issue in linked_issues for label in get_issue_labels(issue) ]
+    to_transfer = [ label for label in all_labels if label not in do_not_transfer ]
+    current_labels = get_issue_labels(pr_number)
+    missing_labels = [ label for label in to_transfer if label not in current_labels ]
+    if not missing_labels:
+        return
+    new_labels = current_labels + missing_labels
+    url = f'https://api.github.com/repos/{repo}/issues/{pr_number}/labels'
+    print(f'adding {missing_labels} to PR #{pr_number}')
+    if not github_token:
+        print('no GITHUB_TOKEN, skipping')
+    else:
+        resp = requests.put(url, headers=headers, data=json.dumps({'labels':new_labels}))
+        resp.raise_for_status()
+
+if __name__ == '__main__':
+    transfer_issue_labels(sys.argv[1])