Fixed wrong return type in BaseCookie.value_encode in http.cookies

[b9788213]

    def value_encode(self, val):
        """real_value, coded_value = value_encode(VALUE)
        Called prior to setting a cookie's value from the dictionary
        representation.  The VALUE is the value being assigned.
        Override this function to modify the behavior of cookies.
        """
        strval = str(val)
        return strval, strval

def value_encode(self, val: _T) -> tuple[_T, str]

value_encode method in BaseCookie in http.cookies have wrong return type. It must be tuple[str, str] because it basically returns two strings.

[github-actions]

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