Resolve an annotation's /Subtype and /Contents when they are indirect objects - #125

A text annotation whose /Contents is an indirect object makes the whole run fail:

AttributeError: 'PDFObjRef' object has no attribute 'startswith'

and an indirect /Subtype stops at assert isinstance(subtype, PSLiteral). Both values may be indirect in a PDF, and _mkannotation() already resolves the other entries it reads (/C, /Rect, /QuadPoints, /T, /NM, the dates) with pdftypes.resolve1(). Now these two are resolved the same way.

Added tests/indirect-objects.pdf (a single text annotation with both values indirect, made with pypdf) and an IndirectObjects test; it fails on main. pytest tests.py, mypy ., flake8 and autopep8 --diff pass.

Used Claude (Opus) to help find and patch this; I reviewed and tested the change