Release workflow opens PR for changelog against master branch

closes: #416

CHANGES/416.feature

@@ -0,0 +1 @@
+Release workflow opens PR against master with the latest changelog.

templates/github/.github/workflows/release.yml.j2

@@ -276,3 +276,20 @@ jobs:
 
       - name: Create release on GitHub
         run: bash .github/workflows/scripts/create_release_from_tag.sh {{ "${{ github.event.inputs.release }}" }}
+
+      - name: Stage changelog for master branch
+        run: python .github/workflows/scripts/stage-changelog-for-master.py {{ "${{ github.event.inputs.release }}" }}
+
+      - name: Create Pull Request for Changelog
+        uses: peter-evans/create-pull-request@v3
+        with:
+          committer: {{  'pulpbot <pulp-infra@redhat.com>' }}
+          author: {{  'pulpbot <pulp-infra@redhat.com>' }}
+          branch: {{ "${{ github.event.inputs.release }}" }}-changelog
+          base: master
+          title: 'Building changelog for {{ "${{ github.event.inputs.release }}" }}'
+          body: '[noissue]'
+          commit-message: |
+            Building changelog for {{ "${{ github.event.inputs.release }}" }}
+            [noissue]
+          delete-branch: true

templates/github/.github/workflows/scripts/stage-changelog-for-master.py.j2

@@ -0,0 +1,51 @@
+{% include 'header.j2' %}
+
+import argparse
+import os
+import textwrap
+
+from git import Repo
+
+
+helper = textwrap.dedent(
+    """\
+        Stage the changelog for a release on master branch.
+
+        Example:
+            $ python .github/workflows/scripts/stage-changelog-for-master.py 3.4.0
+
+    """
+)
+
+parser = argparse.ArgumentParser(formatter_class=argparse.RawTextHelpFormatter, description=helper)
+
+parser.add_argument(
+    "release_version",
+    type=str,
+    help="The version string for the release.",
+)
+
+args = parser.parse_args()
+
+release_version_arg = args.release_version
+
+release_path = os.path.dirname(os.path.abspath(__file__))
+plugin_path = release_path.split("/.github")[0]
+
+print(f"\n\nRepo path: {plugin_path}")
+repo = Repo(plugin_path)
+
+changelog_commit = None
+# Look for a commit with the requested release version
+for commit in repo.iter_commits():
+    if f"Building changelog for {release_version_arg}\n" in commit.message:
+        changelog_commit = commit
+        break
+
+if not changelog_commit:
+    raise RuntimeError("Changelog commit for {release_version_arg} was not found.")
+
+git = repo.git
+git.checkout("origin/master")
+git.cherry_pick(changelog_commit.hexsha)
+git.reset("origin/master")