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/inputhooktk.py

@@ -0,0 +1,23 @@
+# encoding: utf-8
+# Unlike what IPython does, we need to have an explicit inputhook because tkinter handles
+# input hook in the C Source code
+
+#-----------------------------------------------------------------------------
+# Imports
+#-----------------------------------------------------------------------------
+
+from pydev_ipython.inputhook import stdin_ready
+
+#-----------------------------------------------------------------------------
+# Code
+#-----------------------------------------------------------------------------
+
+TCL_DONT_WAIT = 1 << 1
+
+def create_inputhook_tk(app):
+    def inputhook_tk():
+        while app.dooneevent(TCL_DONT_WAIT) == 1:
+            if stdin_ready():
+                break
+        return 0
+    return inputhook_tk