Doesn't work with pycryptodome

[davedoesdev]

pycrypto is dead. pycryptodome is a drop-in replacement.
However, python-jws uses a private member (_RSAobj) of pycrypto:

https://github.com/brianloveswords/python-jws/blob/master/jws/algos.py#L87

which pycryptodome doesn't have.

But pycryptodome does have the RsaKey member in the same place.

So a change something like https://github.com/mpdavis/python-jose/pull/8/files#diff-f5aa2743c17dabc292333673fe591c30R20 is required where the presence of each member is checked for.