refactor: Extract main block to main function

git-tidy

@@ -125,9 +125,8 @@ def get_default_branch() -> str:
 
     return branch_list[selection]
 
-
-if __name__ == "__main__":
-
+def main():
+    """Main function."""
     default_branch = get_default_branch()
     print(f"Switching to {default_branch}")
     run(["git", "checkout", default_branch])
@@ -156,3 +155,6 @@ if __name__ == "__main__":
     for branch in merged_branches:
         if branch[0] != "*":  # This time we want to explicitly exclude the checked-out branch.
             subprocess.run(["git", "branch", "-d", branch.strip()], check=True)
+
+if __name__ == "__main__":
+    main()