py/objslice: Ensure slice is not hashable.

As per https://bugs.python.org/issue408326, the slice object should not be
hashable.  Since MicroPython has an implicit fallback when the unary_op
slot is empty, we need to fill this slot.

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

tests/basics/slice_op.py

@@ -0,0 +1,15 @@
+
+try:
+    t = [][:]
+except:
+    print("SKIP")
+    raise SystemExit
+
+
+# REVISIT: slice comparison operators are not implemented in MicroPython
+
+# test that slice is not hashable, i.e. it can't be used to copy a dict
+try:
+    {}[:] = {}
+except TypeError:
+    print('TypeError')