add types to tkinter invoke methods

[Akuli]

Most invoke methods return whatever the corresponding command of the widget returns, but passed through Tcl. So ints and strings and a few other things are returned as is, but everything else (such as collections.deque) becomes strings:

>>> import collections, tkinter
>>> b = tkinter.Button()
>>> b.invoke()
''
>>> b['command'] = lambda: 123
>>> b.invoke()
123
>>> b['command'] = lambda: collections.deque([1, 2, 3])
>>> b.invoke()
'deque([1, 2, 3])'

This doesn't apply to tkinter.Spinbox. I couldn't get it to return non-empty strings even by setting a custom command. The Tcl documentation doesn't mention anything about its return value, so it makes sense that it is always the empty string; it's similar to None as the default return value in Python.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

porcupine (https://github.com/Akuli/porcupine.git)
+ porcupine/utils.py:694: error: unused "type: ignore" comment