ci: Add pull request assigner workflow

This commit adds the "Pull Request Assigner" workflow that
automatically assigns the reviewers, assignee and labels for a pull
request.

Note that this workflow runs on the `pull_request_target` event in the
context of the base branch using the `MAINTAINERS.yml` from the base
branch.

Signed-off-by: Stephanos Ioannidis <[email protected]>

Author: stephanosio

.github/workflows/assigner.yml

@@ -0,0 +1,32 @@
+name: Pull Request Assigner
+
+on:
+  pull_request_target:
+    branches:
+    - main
+    - v*-branch
+
+jobs:
+  assignment:
+    name: Pull Request Assignment
+    runs-on: ubuntu-20.04
+
+    steps:
+    - name: Install Python dependencies
+      run: |
+        sudo pip3 install -U setuptools wheel pip
+        pip3 install -U PyGithub>=1.55
+
+    - name: Check out source code
+      uses: actions/checkout@v2
+
+    - name: Run assignment script
+      env:
+        GITHUB_TOKEN: ${{ secrets.ZB_GITHUB_TOKEN }}
+      run: |
+        python3 scripts/set_assignees.py \
+          -v \
+          -o ${{ github.event.repository.owner.login }} \
+          -r ${{ github.event.repository.name }} \
+          -M MAINTAINERS.yml \
+          -P ${{ github.event.pull_request.number }}