Update fpdf2 stubs to 2.6.1 (#9546)

stubs/fpdf2/@tests/requirements-stubtest.txt

@@ -0,0 +1 @@
+cryptography

stubs/fpdf2/@tests/stubtest_allowlist.txt

@@ -12,3 +12,8 @@ fpdf.output.signer
 # Argument has default at runtime, but using it raises a TypeError.
 fpdf.FPDF.set_creation_date
 fpdf.fpdf.FPDF.set_creation_date
+
+# Checking the following function crashes stubtest 0.991, but seems to be
+# fixed in later versions.
+fpdf.FPDF.set_encryption
+fpdf.fpdf.FPDF.set_encryption