scripts: Add a supervisor package, to use the XMLRPC Supervisor API.

For many uses, shelling out to `supervisorctl` is going to produce
better error messages.  However, for instances where we wish to parse
the output of `supervisorctl`, using the API directly is less brittle.

scripts/lib/supervisor.py

@@ -0,0 +1,27 @@
+import socket
+from http.client import HTTPConnection
+from typing import Dict, List, Optional, Tuple, Union
+from xmlrpc import client
+
+
+class UnixStreamHTTPConnection(HTTPConnection):
+    def connect(self) -> None:
+        self.sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
+        self.sock.connect(self.host)
+
+
+class UnixStreamTransport(client.Transport, object):
+    def __init__(self, socket_path: str) -> None:
+        self.socket_path = socket_path
+        super(UnixStreamTransport, self).__init__()
+
+    def make_connection(
+        self, host: Union[Tuple[str, Dict[str, str]], str]
+    ) -> UnixStreamHTTPConnection:
+        return UnixStreamHTTPConnection(self.socket_path)
+
+
+def rpc() -> client.ServerProxy:
+    return client.ServerProxy(
+        "http://localhost", transport=UnixStreamTransport("/var/run/supervisor.sock")
+    )