Extending `TraverserVisitor` causes `TypeError: interpreted classes cannot inherit from compiled traits`

Bug Report

Plugin code cannot extend any of the Visitor classes

To Reproduce

from mypy.nodes import IndexExpr, MypyFile
from mypy.plugin import Plugin
from mypy.traverser import TraverserVisitor


def plugin(_: str) -> type[Plugin]:
    return ReproducerPlugin


class ReproducerPlugin(Plugin):
    def get_additional_deps(self, file: MypyFile) -> list[tuple[int, str, int]]:
        class IndexVisitor(TraverserVisitor):
            def visit_index_expr(self, ie: IndexExpr, /) -> None:
                print(ie)

        file.accept(IndexVisitor())

Expected Behavior
It not crash

Actual Behavior

[ERROR   ] An un-handled exception was caught by Salt's global exception handler:
TypeError: interpreted classes cannot inherit from compiled traits
Traceback (most recent call last):
  File "venv/bin/mypy", line 8, in <module>
    sys.exit(console_entry())
             ~~~~~~~~~~~~~^^
  File "venv/lib/python3.13/site-packages/mypy/__main__.py", line 15, in console_entry
    main()
    ~~~~^^
  File "mypy/main.py", line 127, in main
  File "mypy/main.py", line 211, in run_build
  File "mypy/build.py", line 191, in build
  File "mypy/build.py", line 267, in _build
  File "mypy/build.py", line 2886, in dispatch
  File "mypy/build.py", line 3072, in load_graph
  File "mypy/build.py", line 1994, in __init__
  File "mypy/build.py", line 2292, in compute_dependencies
  File "mypy/plugin.py", line 845, in get_additional_deps
  File "plugin/__init__.py", line 16, in get_additional_deps
    file.accept(IndexVisitor())
                ~~~~~~~~~~~~^^
TypeError: interpreted classes cannot inherit from compiled traits

Your Environment

LOG:  Mypy Version:           1.17.0
LOG:  Config File:            pyproject.toml
LOG:  Configured Executable:  venv/bin/python3.13
LOG:  Current Executable:     venv/bin/python3.13
LOG:  Cache Dir:              .mypy_cache
LOG:  Compiled:               True
LOG:  Exclude:                []
LOG:  Found source:           BuildSource(path='test.py', module='test', has_text=False, base_dir='.', followed=False)
LOG:  Metadata abandoned for test: options differ
LOG:  Metadata not found for test
LOG:  Parsing test.py (test)
LOG:  Build finished in 0.013 seconds with 1 modules, and 0 errors

from pyproject.toml:

[tool.mypy]
plugins = ["pydantic.mypy", "plugin"]
strict = true
namespace_packages = true
mypy_path = ".mypy-stubs/"
scripts_are_modules = true

• Python version used: 3.13

まず、提供された Python 3.13 の再現プログラムをコンパイル済みの mypy で実行し、次に mypy/traverser.py の TraverserVisitor と、mypy/plugin.py に示されているプラグイン読み込み経路を調査します。解釈されたサブクラスの初期化に失敗する理由を追跡し、再現プログラムが TypeError なしで完了することを確認します。