Add return type to _tkinter.TkappType.interpaddr()

[Akuli]

I needed this for a project.

How to verify that this is correct:

>>> import tkinter, _tkinter
>>> _tkinter.TkappType
<class '_tkinter.tkapp'>
>>> root = tkinter.Tk()
>>> type(root.tk)
<class '_tkinter.tkapp'>
>>> root.tk.interpaddr()
754380928

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉