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/common/stacks.py

@@ -0,0 +1,62 @@
+# Copyright (c) Microsoft Corporation. All rights reserved.
+# Licensed under the MIT License. See LICENSE in the project root
+# for license information.
+
+"""Provides facilities to dump all stacks of all threads in the process.
+"""
+
+import os
+import sys
+import time
+import threading
+import traceback
+
+from debugpy.common import log
+
+
+def dump():
+    """Dump stacks of all threads in this process, except for the current thread."""
+
+    tid = threading.current_thread().ident
+    pid = os.getpid()
+
+    log.info("Dumping stacks for process {0}...", pid)
+
+    for t_ident, frame in sys._current_frames().items():
+        if t_ident == tid:
+            continue
+
+        for t in threading.enumerate():
+            if t.ident == tid:
+                t_name = t.name
+                t_daemon = t.daemon
+                break
+        else:
+            t_name = t_daemon = "<unknown>"
+
+        stack = "".join(traceback.format_stack(frame))
+        log.info(
+            "Stack of thread {0} (tid={1}, pid={2}, daemon={3}):\n\n{4}",
+            t_name,
+            t_ident,
+            pid,
+            t_daemon,
+            stack,
+        )
+
+    log.info("Finished dumping stacks for process {0}.", pid)
+
+
+def dump_after(secs):
+    """Invokes dump() on a background thread after waiting for the specified time."""
+
+    def dumper():
+        time.sleep(secs)
+        try:
+            dump()
+        except:
+            log.swallow_exception()
+
+    thread = threading.Thread(target=dumper)
+    thread.daemon = True
+    thread.start()