How to annotate function arguments hidden in *args?

Question

I am testing mypy in one of my projects to see if I will like or not. I know only the basics so far. Here is a little problem I am unable to solve.

I need a function with positional-only parameters. In Python 3.8+:

def func(self, arg: int, /, **data):

(This allows to use self=something and arg=something in the data, if you're curious why.)

To make it work also in Python 3.7 I had to write:

def func(*args, **data):
    self, arg = args

And it works fine, but seems to confuse the mypy, it complains about parameter types in calls of this func.

How can I annotatate, that arg[1] is an int?

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Update:

I had some partial success with typing.overload.

A single @overload generates this error: "Single overload definition, multiple required" and when I write it twice: "Overloaded function signature 2 will never be matched" but the calls to that method are now checked OK.