Only support ==/!= int on unscoped enums

This makes the Python interface mirror the C++ interface:
pybind11-exported scoped enums aren't directly comparable to the
underlying integer values.
diff --git a/example/example-constants-and-functions.ref b/example/example-constants-and-functions.ref
index f824806..4805631 100644
--- a/example/example-constants-and-functions.ref
+++ b/example/example-constants-and-functions.ref
@@ -12,6 +12,8 @@
 None
 test_ecenum(ECMyEnum::Three)
 test_ecenum(ECMyEnum::Two)
+Good: caught expected TypeError exceptions for scoped enum ==/!= int comparisons
+Good: no TypeError exception for unscoped enum ==/!= int comparisions
 enum->integer = 2
 integer->enum = EMyEnumeration.ESecondEntry
 A constant = 14