chore(gh): Automate release creation

.github/workflows/post-release.yml

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+name: post-release
+on:
+  push:
+    tags:
+    - "v*"
+jobs:
+  create-release:
+    name: create-release
+    runs-on: ubuntu-latest
+    outputs:
+      upload_url: ${{ steps.release.outputs.upload_url }}
+      release_version: ${{ env.RELEASE_VERSION }}
+    steps:
+    - name: Get the release version from the tag
+      shell: bash
+      if: env.RELEASE_VERSION == ''
+      run: |
+        # See: https://github.community/t5/GitHub-Actions/How-to-get-just-the-tag-name/m-p/32167/highlight/true#M1027
+        echo "RELEASE_VERSION=${GITHUB_REF#refs/tags/}" >> $GITHUB_ENV
+        echo "version is: ${{ env.RELEASE_VERSION }}"
+    - name: Checkout repository
+      uses: actions/checkout@v2
+      with:
+        fetch-depth: 1
+    - name: Generate Release Notes
+      run: |
+        ./.github/workflows/release-notes.py --tag ${{ env.RELEASE_VERSION }} --output notes-${{ env.RELEASE_VERSION }}.md
+        cat notes-${{ env.RELEASE_VERSION }}.md
+    - name: Create GitHub release
+      id: release
+      uses: actions/create-release@v1
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      with:
+        tag_name: ${{ env.RELEASE_VERSION }}
+        release_name: ${{ env.RELEASE_VERSION }}
+        body_path: notes-${{ env.RELEASE_VERSION }}.md

.github/workflows/release-notes.py

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+#!/usr/bin/env python3
+
+import argparse
+import re
+import pathlib
+import sys
+
+
+_STDIO = pathlib.Path("-")
+
+
+def main():
+    parser = argparse.ArgumentParser()
+    parser.add_argument("-i", "--input", type=pathlib.Path, default="CHANGELOG.md")
+    parser.add_argument("--tag", required=True)
+    parser.add_argument("-o", "--output", type=pathlib.Path, required=True)
+    args = parser.parse_args()
+
+    if args.input == _STDIO:
+        lines = sys.stdin.readlines()
+    else:
+        with args.input.open() as fh:
+            lines = fh.readlines()
+    version = args.tag.lstrip("v")
+
+    note_lines = []
+    for line in lines:
+        if line.startswith("## ") and version in line:
+            note_lines.append(line)
+        elif note_lines and line.startswith("## "):
+            break
+        elif note_lines:
+            note_lines.append(line)
+
+    notes = "".join(note_lines).strip()
+    if args.output == _STDIO:
+        print(notes)
+    else:
+        args.output.write_text(notes)
+
+
+if __name__ == "__main__":
+    main()