OKPKey.__delitem__() allows creation of invalid COSE OKP Key

[steffen-kiess]

The OKPKey.__delitem__() function has a bug in the if condition. Currently the code

import pycose.keys.curves
key = pycose.keys.OKPKey.generate_key(pycose.keys.curves.Ed25519)
del key[pycose.keys.keyparam.OKPKpX]
del key[pycose.keys.keyparam.OKPKpD]
print(key)

will run and print

<COSE_Key(OKPKey): {'OKPKpCurve': 'Ed25519', 'KpKty': 'KtyOKP'}>

The if in line 249

pycose/pycose/keys/okp.py

Line 249 in 1458ddf

if self._key_transform(key) == OKPKpX and OKPKpD not in self.store:

should be an elif, otherwise the first if is ignored.