Refactor GitHub Actions workflow and add scripts for versioning and release notes extraction

.github/scripts/get_version.py

@@ -0,0 +1,8 @@
+from pathlib import Path
+import re
+
+text = Path("CHANGELOG.md").read_text(encoding="utf-8")
+m = re.search(r"^##\s+FocusGram\s+([0-9]+\.[0-9]+\.[0-9]+)\s*$", text, re.M)
+if not m:
+    raise SystemExit("Could not find a top changelog heading like: ## FocusGram 2.0.0")
+print(m.group(1))