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_packaging.py

@@ -0,0 +1,48 @@
+# Copyright (c) Microsoft Corporation. All rights reserved.
+# Licensed under the MIT License. See LICENSE in the project root
+# for license information.
+
+from . import VENDORED_ROOT
+from ._util import cwd, iter_all_files
+
+
+INCLUDES = [
+    'setup_pydevd_cython.py',
+]
+
+
+def iter_files():
+    # From the root of pydevd repo, we want only scripts and
+    # subdirectories that constitute the package itself (not helper
+    # scripts, tests etc). But when walking down into those
+    # subdirectories, we want everything below.
+
+    with cwd(VENDORED_ROOT):
+        return iter_all_files('pydevd', prune_dir, exclude_file)
+
+
+def prune_dir(dirname, basename):
+    if basename == '__pycache__':
+        return True
+    elif dirname != 'pydevd':
+        return False
+    elif basename.startswith('pydev'):
+        return False
+    elif basename.startswith('_pydev'):
+        return False
+    return True
+
+
+def exclude_file(dirname, basename):
+    if dirname == 'pydevd':
+        if basename in INCLUDES:
+            return False
+        elif not basename.endswith('.py'):
+            return True
+        elif 'pydev' not in basename:
+            return True
+        return False
+
+    if basename.endswith('.pyc'):
+        return True
+    return False