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

tests/test_image_input.py

@@ -87,4 +87,4 @@ def test_jpeg_in_jpeg_out(resources, outpdf):
         'tests/plugins/tesseract_noop.py',
     )
     with pikepdf.open(outpdf) as pdf:
-        assert next(pdf.pages[0].images.values()).Filter == pikepdf.Name.DCTDecode
+        assert next(iter(pdf.pages[0].images.values())).Filter == pikepdf.Name.DCTDecode