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/_version.py

@@ -0,0 +1,21 @@
+
+# This file was generated by 'versioneer.py' (0.23) from
+# revision-control system data, or from the parent directory name of an
+# unpacked source archive. Distribution tarballs contain a pre-generated copy
+# of this file.
+
+import json
+
+version_json = '''
+{
+ "date": "2023-09-05T13:20:31-0700",
+ "dirty": false,
+ "error": null,
+ "full-revisionid": "5723ed6b1a058145d727ae5e4b065bd65072f35b",
+ "version": "1.7.0"
+}
+'''  # END VERSION_JSON
+
+
+def get_versions():
+    return json.loads(version_json)