[fpdf2] fix link type to support internal links

[mschoettle]

Methods with the link argument can take an internal link (created via add_link which returns an int) or an external URL as per their docstring.

The type hint only allows for str.

link already has the correct type hint:

typeshed/stubs/fpdf2/fpdf/fpdf.pyi

Lines 343 to 345 in a94c927

	def link(
	self, x: float, y: float, w: float, h: float, link: str | int, alt_text: str | None = None, border_width: int = 0
	) -> AnnotationDict: ...

add_link returns an int:

typeshed/stubs/fpdf2/fpdf/fpdf.pyi

Line 341 in a94c927

def add_link(self, y: float = 0, x: float = 0, page: int = -1, zoom: float | Literal["null"] = "null") -> int: ...

This PR changes the type hint to str | int.

[github-actions]

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

[srittau]

Thanks!