py/objdict: Fix __hash__ for dict_view types.

This adds a unary_op implementation for the dict_view type that makes
the implementation of `hash()` for these types compatible with CPython.

Signed-off-by: David Lechner <david@pybricks.com>

tests/basics/dict_views.py

@@ -18,4 +18,22 @@ try:
 except TypeError:
     print('TypeError')
 
+# keys dict_view is not hashable
+
+try:
+    hash({}.keys())
+except TypeError:
+    print('TypeError')
+
+# values dict_view is hashable
+
+print(type(hash({}.values())))
+
+# items dict_view is not hashable
+
+try:
+    hash({}.items())
+except TypeError:
+    print('TypeError')
+
 # set operations still to come