Fail lint CI if the PR doesn't target main (psf#3477)

Let's skip the check if we're running on a fork just in case someone
opens a PR against a branch on said fork as part of a PR review
upstream.

.github/workflows/lint.yml

@@ -16,6 +16,13 @@ jobs:
     steps:
       - uses: actions/checkout@v3
 
+      - name: Assert PR target is main
+        if: github.event_name == 'pull_request' && github.repository == 'psf/black'
+        run: |
+          if [ "$GITHUB_BASE_REF" != "main" ]; then
+              echo "::error::PR targeting '$GITHUB_BASE_REF', please refile targeting 'main'." && exit 1
+          fi
+
       - name: Set up latest Python
         uses: actions/setup-python@v4
         with: