[fpdf2] Update to 2.7.1 (#9978)

stubs/fpdf2/@tests/stubtest_allowlist.txt

@@ -1,7 +1,3 @@
-# The "dest" argument is unused and will be removed.
-fpdf.FPDF.output
-fpdf.fpdf.FPDF.output
-
 # Argument has default at runtime, but using it raises a TypeError.
 fpdf.FPDF.set_creation_date
 fpdf.fpdf.FPDF.set_creation_date
@@ -13,3 +9,8 @@ fpdf._fonttools_shims
 # fixed in later versions.
 fpdf.FPDF.set_encryption
 fpdf.fpdf.FPDF.set_encryption
+
+# Runtime has some internal arguments.
+fpdf.syntax.build_obj_dict
+fpdf.ViewerPreferences.serialize
+fpdf\.[a-z]+\.[A-Za-z]+\.serialize