cbor2: Improve decoder to pass Ruff F821 undefined-name.

These were probably intentional missing names, however raising
NotImplementedError or KeyError is more explicit than trying to call an
unknown function.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

python-ecosys/cbor2/cbor2/decoder.py

@@ -160,7 +160,7 @@ def decode_simple_value(decoder):
 
 def decode_float16(decoder):
     payload = decoder.read(2)
-    return unpack_float16(payload)
+    raise NotImplementedError  # no float16 unpack function
 
 
 def decode_float32(decoder):
@@ -185,7 +185,7 @@ special_decoders = {
     20: lambda self: False,
     21: lambda self: True,
     22: lambda self: None,
-    23: lambda self: undefined,
+    # 23 is undefined
     24: decode_simple_value,
     25: decode_float16,
     26: decode_float32,