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/force_pydevd.py

@@ -0,0 +1,81 @@
+# Copyright (c) Microsoft Corporation. All rights reserved.
+# Licensed under the MIT License. See LICENSE in the project root
+# for license information.
+
+from importlib import import_module
+import os
+import warnings
+
+from . import check_modules, prefix_matcher, preimport, vendored
+
+# Ensure that pydevd is our vendored copy.
+_unvendored, _ = check_modules('pydevd',
+                               prefix_matcher('pydev', '_pydev'))
+if _unvendored:
+    _unvendored = sorted(_unvendored.values())
+    msg = 'incompatible copy of pydevd already imported'
+    # raise ImportError(msg)
+    warnings.warn(msg + ':\n {}'.format('\n  '.join(_unvendored)))
+
+# If debugpy logging is enabled, enable it for pydevd as well
+if "DEBUGPY_LOG_DIR" in os.environ:
+    os.environ[str("PYDEVD_DEBUG")] = str("True")
+    os.environ[str("PYDEVD_DEBUG_FILE")] = os.environ["DEBUGPY_LOG_DIR"] + str("/debugpy.pydevd.log")
+
+# Disable pydevd frame-eval optimizations only if unset, to allow opt-in.
+if "PYDEVD_USE_FRAME_EVAL" not in os.environ:
+    os.environ[str("PYDEVD_USE_FRAME_EVAL")] = str("NO")
+
+# Constants must be set before importing any other pydevd module
+# # due to heavy use of "from" in them.
+with warnings.catch_warnings():
+    warnings.simplefilter("ignore", category=DeprecationWarning)
+    with vendored('pydevd'):
+        pydevd_constants = import_module('_pydevd_bundle.pydevd_constants')
+# We limit representation size in our representation provider when needed.
+pydevd_constants.MAXIMUM_VARIABLE_REPRESENTATION_SIZE = 2 ** 32
+
+# Now make sure all the top-level modules and packages in pydevd are
+# loaded.  Any pydevd modules that aren't loaded at this point, will
+# be loaded using their parent package's __path__ (i.e. one of the
+# following).
+with warnings.catch_warnings():
+    warnings.simplefilter("ignore", category=DeprecationWarning)
+    preimport('pydevd', [
+        '_pydev_bundle',
+        '_pydev_runfiles',
+        '_pydevd_bundle',
+        '_pydevd_frame_eval',
+        'pydev_ipython',
+        'pydevd_plugins',
+        'pydevd',
+    ])
+
+# When pydevd is imported it sets the breakpoint behavior, but it needs to be
+# overridden because by default pydevd will connect to the remote debugger using
+# its own custom protocol rather than DAP.
+import pydevd  # noqa
+import debugpy  # noqa
+
+
+def debugpy_breakpointhook():
+    debugpy.breakpoint()
+
+
+pydevd.install_breakpointhook(debugpy_breakpointhook)
+
+# Ensure that pydevd uses JSON protocol
+from _pydevd_bundle import pydevd_constants
+from _pydevd_bundle import pydevd_defaults
+pydevd_defaults.PydevdCustomization.DEFAULT_PROTOCOL = pydevd_constants.HTTP_JSON_PROTOCOL
+
+# Enable some defaults related to debugpy such as sending a single notification when
+# threads pause and stopping on any exception.
+pydevd_defaults.PydevdCustomization.DEBUG_MODE = 'debugpy-dap'
+
+# This is important when pydevd attaches automatically to a subprocess. In this case, we have to
+# make sure that debugpy is properly put back in the game for users to be able to use it.
+pydevd_defaults.PydevdCustomization.PREIMPORT = '%s;%s' % (
+    os.path.dirname(os.path.dirname(debugpy.__file__)), 
+    'debugpy._vendored.force_pydevd'
+)