PdfPage.get_objects(): don't register objects as kids

This was especially problematic as weakrefs are not cleaned up when the
object in question is closed/collected, so we potentially store many
dead pointers.
Imagine a caller invoking get_objects() repeatedly for iterating and a
page handle living for a long time afterwards - that somewhat resembles
a memory leak.

src/pypdfium2/_helpers/page.py

@@ -283,8 +283,8 @@ def get_objects(self, filter=None, max_depth=2, form=None, level=0):
             if raw_obj is None:
                 raise PdfiumError("Failed to get page object.")
 
+            # Not a child object, because the lifetime of pageobjects that are part of a page is managed by pdfium. The .page reference is enough to keep the parent alive, unless the caller explicitly closes it (which may not merit storing countless of weakrefs).
             helper_obj = PdfObject(raw_obj, page=self, pdf=self.pdf, level=level)
-            self._add_kid(helper_obj)
             if not filter or helper_obj.type in filter:
                 yield helper_obj