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/_vendored/pydevd/pydev_ipython/inputhookgtk3.py

@@ -0,0 +1,35 @@
+# encoding: utf-8
+"""
+Enable Gtk3 to be used interacive by IPython.
+
+Authors: Thomi Richards
+"""
+#-----------------------------------------------------------------------------
+# Copyright (c) 2012, the IPython Development Team.
+#
+# Distributed under the terms of the Modified BSD License.
+#
+# The full license is in the file COPYING.txt, distributed with this software.
+#-----------------------------------------------------------------------------
+
+#-----------------------------------------------------------------------------
+# Imports
+#-----------------------------------------------------------------------------
+
+from gi.repository import Gtk, GLib  # @UnresolvedImport
+
+#-----------------------------------------------------------------------------
+# Code
+#-----------------------------------------------------------------------------
+
+def _main_quit(*args, **kwargs):
+    Gtk.main_quit()
+    return False
+
+
+def create_inputhook_gtk3(stdin_file):
+    def inputhook_gtk3():
+        GLib.io_add_watch(stdin_file, GLib.IO_IN, _main_quit)
+        Gtk.main()
+        return 0
+    return inputhook_gtk3