[reportlab] Add more annotations for pdfbase

[donbarbos]

No description provided.

[srittau]

You've used LiteralString as return type for various functions. I think we should be more conservative and just use str. While the strings are constructed according to specific rules, LiteralStrings should only be used for strings provided by the user or directly constructed from other LiteralStrings.

[github-actions]

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