Support pikepdf 6.0+

[sjustas]

PikePdf 6.0.0 introduced stricter checks against copying objects from another pdf.

This PR fixes merging test failure, see https://github.com/zopefoundation/z3c.rml/actions/runs/3077053438

Error in test runTest (z3c.rml.tests.test_rml.tag-mergePage)
Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.10.6/x64/lib/python3.10/unittest/case.py", line 59, in testPartExecutor
    yield
  File "/opt/hostedtoolcache/Python/3.10.6/x64/lib/python3.10/unittest/case.py", line 591, in run
    self._callTestMethod(testMethod)
  File "/opt/hostedtoolcache/Python/3.10.6/x64/lib/python3.10/unittest/case.py", line 549, in _callTestMethod
    method()
  File "/home/runner/work/z3c.rml/z3c.rml/src/z3c/rml/tests/test_rml.py", line 69, in runTest
    rml2pdf.go(self._inPath, self._outPath)
  File "/home/runner/work/z3c.rml/z3c.rml/src/z3c/rml/rml2pdf.py", line 77, in go
    doc.process(outputFile)
  File "/home/runner/work/z3c.rml/z3c.rml/src/z3c/rml/document.py", line 764, in process
    tempOutput = processor.process(tempOutput)
  File "/home/runner/work/z3c.rml/z3c.rml/src/z3c/rml/page.py", line 62, in process
    mergePage(toMerge, page, input1, name)
  File "/home/runner/work/z3c.rml/z3c.rml/src/z3c/rml/page.py", line 41, in mergePage
    mainPage.MediaBox = layerPage.MediaBox
  File "/home/runner/work/z3c.rml/z3c.rml/.tox/py310/lib/python3.10/site-packages/pikepdf/_methods.py", line 1112, in __setattr__
    setattr(self.obj, name, value)
pikepdf._qpdf.ForeignObjectError: Attempting to add an object from a different pikepdf.Pdf. Use pikepdf.copy_foreign to add objects from another file.

Added some tests to increase coverage.