Skip /Annots entries that have no valid /Subtype - #128

When a page's /Annots array refers to a dictionary that isn't an annotation, pdfannots stops with:

  File "pdfannots/__init__.py", line 57, in _mkannotation
    assert isinstance(subtype, PSLiteral)
AssertionError

I hit this with tests/resources/test_2861.pdf from PyMuPDF (from pymupdf/PyMuPDF#2861), a published paper. Its first page's /Annots lists a link's action dictionary (/S /URI) and its border array next to the two /Link annotations. The border array already gives an empty dict and is skipped, but the action dictionary reaches _mkannotation() without a /Subtype.

_mkannotation() now logs a warning and returns None for such an entry, like it does for an unsupported subtype, and the rest of the page is read.

I didn't add the paper itself as a test file (copyright, and an author's e-mail address in it). tests/annots-entries-without-subtype.pdf has the same structure, made with pypdf: a border array, an action dictionary and a Text annotation in /Annots. The AnnotsEntriesWithoutSubtype test checks that the Text annotation is extracted; it fails on main. pytest tests.py, mypy ., flake8 pdfannots tests.py and autopep8 --diff pass.

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