stubgen: missing `__str__` method when the signature is different from default

Bug Report
When generating a stub with classes that define __str__ with extra parameters, __str__ is missing from the generated stub.

(A clear and concise description of what the bug is.)

To Reproduce

stubgen -p PyInstaller.utils.win32.versioninfo 

Expected Behavior

Classes in the generated PyInstaller.utils.win32.versioninfo stub should define def __str__(self, indent: str = "") -> str: ...

Actual Behavior
It's missing. Which results in a stubtest failure that has to be fixed manually.

Your Environment

• Mypy version used: mypy 1.0.0 (compiled: yes)
• Mypy command-line flags: N/A (stubgen, see above)
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: Python 3.9.13

まず stubgen -p PyInstaller.utils.win32.versioninfo で問題を再現し、次に __str__ メソッドを処理する stubgen のエントリーポイントを追跡します。対象クラスの生成されたスタブに def __str__(self, indent: str = "") -> str: ... が含まれ、報告された stubtest の失敗を回避できれば完了です。