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/qt.py

@@ -0,0 +1,23 @@
+""" A Qt API selector that can be used to switch between PyQt and PySide.
+
+This uses the ETS 4.0 selection pattern of:
+PySide first, PyQt with API v2. second.
+
+Do not use this if you need PyQt with the old QString/QVariant API.
+"""
+
+import os
+
+from pydev_ipython.qt_loaders import (load_qt, QT_API_PYSIDE,
+                                         QT_API_PYQT, QT_API_PYQT5)
+
+QT_API = os.environ.get('QT_API', None)
+if QT_API not in [QT_API_PYSIDE, QT_API_PYQT, QT_API_PYQT5, None]:
+    raise RuntimeError("Invalid Qt API %r, valid values are: %r, %r" %
+                       (QT_API, QT_API_PYSIDE, QT_API_PYQT, QT_API_PYQT5))
+if QT_API is None:
+    api_opts = [QT_API_PYSIDE, QT_API_PYQT, QT_API_PYQT5]
+else:
+    api_opts = [QT_API]
+
+QtCore, QtGui, QtSvg, QT_API = load_qt(api_opts)