Special values that are reserved in the RFC cause a SystemError exception in decoder

[Sekenre]

example:

>>> import cbor2
>>> cbor2.loads(b'\xfc')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
SystemError: <built-in function loads> returned NULL without setting an error

With the pure python module it raises a KeyError exception.

The value corresponds to a reserved Major Type 7 special value.

[Sekenre]

SystemError also happens when unpacking the payload of Decimals (tags 4 and 5) and it's not a pair.