Fix test failures on pikepdf 3.2.0 + pybind11 2.8.0

When compiled without pybind11 2.8.0, pikepdf supplies a shim to implement
pikepdf._ObjectMapping.values() which has subtly different semantics
from a true dict-like objects; in particular it supports
next(objectmap.values())
where a standard dict requires
next(iter(objectmap.values()).

pybind11 2.8.0 now implements .values() properly, meaning some misuses of
protocol  in ocrmypdf fail.

If pybind11 < 2.8.0, pikepdf will
continue to offer its shim. If pybind11 >= 2.8.0, pikepdf does not add its shim.

Consequently no changes were needed in pikepdf.

Closes #843

src/ocrmypdf/optimize.py

@@ -454,7 +454,7 @@ def _transcode_png(pike: Pdf, filename: Path, xref: Xref) -> bool:
         img2pdf.convert(fspath(filename), outputstream=f)
 
     with Pdf.open(output) as pdf_image:
-        foreign_image = next(pdf_image.pages[0].images.values())
+        foreign_image = next(iter(pdf_image.pages[0].images.values()))
         local_image = pike.copy_foreign(foreign_image)
 
         im_obj = pike.get_object(xref, 0)