tests/cpydiff: Add CPy diff-test for using dict.keys() as a set.

See issue #5493.

tests/cpydiff/types_dict_keys_set.py

@@ -0,0 +1,7 @@
+"""
+categories: Types,dict
+description: Dictionary keys view does not behave as a set.
+cause: Not implemented.
+workaround: Explicitly convert keys to a set before using set operations.
+"""
+print({1:2, 3:4}.keys() & {1})