Generate and print post-release community forum post text in finish_release.py (#10659)

Item 5 of https://github.com/certbot/certbot/issues/10600

When this is merged, the setup section of the [release
process](https://github.com/EFForg/certbot-misc/wiki/The-Mystical-Release-Process)
should be modified.
- `gh` should be added to os packages for mac and debian
- A new step should be added: "Run `gh auth login` and log into a GitHub
account." Technically any account should work here.

The contents of the step saying to post to the community forum should be
shortened to say something like "copy the output from the terminal." We
could add the link to the client-dev tag here, but personally I think
it's easiest to just keep it in the release instructions.

tools/finish_release.py

@@ -176,10 +176,34 @@ def fetch_version_number(major_version=None):
             return version
     raise ValueError('Release not found on Azure Pipelines!')
 
+def generate_community_forum_post(version: str):
+    print('Generating announcement text for community forum post')
+
+    cmd = f"gh release view v{version} --json body -t {{{{.body}}}}".split()
+
+    try:
+        process = subprocess.run(cmd, check=True, stdout=subprocess.PIPE, universal_newlines=True)
+    except (subprocess.CalledProcessError, OSError):
+        print("Generating announcement text failed.")
+        sys.exit(1)
+
+    changelog = process.stdout
+
+    print('Subject:')
+    print()
+    print(f'Certbot {version} Release')
+    print()
+    print('Contents:')
+    print()
+    print(f'Certbot {version} has just been released. The changelog for the release is:')
+    print()
+    print(changelog)
+
 def main(args):
     parsed_args = parse_args(args)
     version = fetch_version_number()
     promote_snaps(ALL_SNAPS, 'beta', version)
+    generate_community_forum_post(version)
 
 if __name__ == "__main__":
     main(sys.argv[1:])