fix: Python 3.6 compatibility for tests

- Replace walrus operator (:=) with traditional assignment in config.py
- Replace datetime.fromisoformat() with strptime() for Python 3.6
- Replace subprocess capture_output and text params with PIPE and universal_newlines
- All tests now pass on Python 3.6.9

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

tddai/workspace.py

@@ -83,7 +83,7 @@ class WorkspaceManager:
                 issue_title=issue_data['title'],
                 issue_body=issue_data['body'],
                 issue_state=issue_data['state'],
-                created_at=datetime.fromisoformat(issue_data['created_at']),
+                created_at=datetime.strptime(issue_data['created_at'].replace('Z', '').split('.')[0], '%Y-%m-%dT%H:%M:%S'),
                 workspace_dir=self.config.workspace_dir
             )
         except (json.JSONDecodeError, KeyError, ValueError) as e: