Vendor debugpy for VS Code attach debugging

Bundles debugpy 1.7.0 directly in python/ so "Maya: Attach (debugpy)"
works with no per-machine pip install step. Installed via Maya 2022's
own pip so it resolved a version actually compatible with Python 3.7
(Maya 2022's interpreter), then verified import + listen() succeeds
under all three target Maya Python versions (3.7/3.9/3.10).

Also fixes a latent __file__-under-exec() bug in
start_debug_server.py (same pitfall as Maya's own plugin loader,
never hit until this exercised it) and corrects the README's Maya
Python version claim -- 2022 ships Python 3.7, not 3.9 as previously
stated, which was never independently verified until now.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

python/debugpy/launcher/__init__.py

@@ -0,0 +1,32 @@
+# Copyright (c) Microsoft Corporation. All rights reserved.
+# Licensed under the MIT License. See LICENSE in the project root
+# for license information.
+
+__all__ = []
+
+
+adapter_host = None
+"""The host on which adapter is running and listening for incoming connections
+from the launcher and the servers."""
+
+channel = None
+"""DAP message channel to the adapter."""
+
+
+def connect(host, port):
+    from debugpy.common import log, messaging, sockets
+    from debugpy.launcher import handlers
+
+    global channel, adapter_host
+    assert channel is None
+    assert adapter_host is None
+
+    log.info("Connecting to adapter at {0}:{1}", host, port)
+
+    sock = sockets.create_client()
+    sock.connect((host, port))
+    adapter_host = host
+
+    stream = messaging.JsonIOStream.from_socket(sock, "Adapter")
+    channel = messaging.JsonMessageChannel(stream, handlers=handlers)
+    channel.start()