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

Beginne damit, das Problem mit stubgen -p PyInstaller.utils.win32.versioninfo zu reproduzieren, und verfolge dann den stubgen-Einstiegspunkt, der __str__-Methoden verarbeitet. Als abgeschlossen gilt die Aufgabe, wenn der generierte Stub für die betroffenen Klassen def __str__(self, indent: str = "") -> str: ... enthält und den gemeldeten stubtest-Fehler vermeidet.