make tkinter.Event generic and add missing type hints to bind methods

[Akuli]

• tkinter.Event is now generic, so tkinter.Event[tkinter.Entry] denotes an event coming from an Entry widget (e.g. user typed text to entry or clicked the entry).
• Bind callbacks are now required to return Literal['break'] or None. At runtime, almost any return value is acceptable (more precisely, anything that could be converted to a Tcl object), and anything except the string 'break' would be treated the same. This breaks code that declares the return type as Optional[str]. If this is too bad, I can change Literal['break'] to str.
• Plain tkinter.Event without specifying a widget type is supported for backwards compatibility, but unfortunately that's equivalent to tkinter.Event[tkinter.Any] rather than tkinter.Event[tkinter.Misc], so event.widget will in this case be Any-typed. It was previously typed Misc rather than any. This shouldn't cause backwards compatibility issues because nobody is using tkinter with mypy --strict (that just doesn't work with what's currently in typeshed).
• bind() return value and other arguments are no longer Any typed.
• Similar changes were done to all binding methods.

[Akuli]

there's conversation related to this in #4200

[Akuli]

Currently CI checks are failing because these overloads overlap:

    @overload
    def bind(
        self,
        sequence: Optional[str] = ...,
        func: Optional[Callable[[Event[Misc]], Optional[Literal["break"]]]] = ...,
        add: Optional[bool] = ...,
    ) -> str: ...
    @overload
    def bind(self, sequence: Optional[str] = ..., func: str = ..., add: Optional[bool] = ...) -> None: ...

I believe widget.bind('<Button-1>') is an example of an overlapping call. I want that to return str. How should I do this?

[srittau]

As far as I understand, you would want to use the following overload (i.e. no default for func):

    @overload
    def bind(self, sequence: Optional[str] = ..., func: str, add: Optional[bool] = ...) -> None: ...

Since this isn't possible, you need to use two overloads:

    @overload
    def bind(self, sequence: Optional[str], func: str, add: Optional[bool] = ...) -> None: ...
    @overload
    def bind(self, *, func: str, add: Optional[bool] = ...) -> None: ...

[Akuli]

@Hawk777 What's your opinion on Optional[Literal['break']] vs Optinal[str] and backwards compatibility?

[Hawk777]

Personally, I like Optional[Literal['break']]. It’s not like linters in general promise to never add new warnings! Sure, it will make existing code fail mypy, but it won’t break actual execution of existing code because execution ignores the type hints anyway.

[srittau]

I have no experience with tk, but the changes look sensible to me.