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/tests_mainloop/gui-tk.py

@@ -0,0 +1,33 @@
+#!/usr/bin/env python
+"""Simple Tk example to manually test event loop integration.
+
+To run this:
+1) Enable the PyDev GUI event loop integration for tk
+2) do an execfile on this script
+3) ensure you have a working GUI simultaneously with an
+   interactive console
+"""
+
+if __name__ == '__main__':
+    
+    try:
+        from Tkinter import *
+    except:
+        # Python 3
+        from tkinter import *
+    
+    class MyApp:
+    
+        def __init__(self, root):
+            frame = Frame(root)
+            frame.pack()
+    
+            self.button = Button(frame, text="Hello", command=self.hello_world)
+            self.button.pack(side=LEFT)
+    
+        def hello_world(self):
+            print("Hello World!")
+    
+    root = Tk()
+    
+    app = MyApp(root)