Discontinue pycrypto support

[lukpueh]

pycrypto which we provided as crypto backend alongside pyca/cryptography is no longer maintained (tip of master 3 years old and usage discouraged in issue tracker).

We should probably replace it with the drop-in replacement pycryptodome or drop it altogether?

[vladimir-v-diaz]

It makes sense to switch to pycryptodome, or similar, if PyCrypto is truly no longer maintained (rather than stable and no longer accepting feature requests).

I would rather provide multiple library options rather than relying on a single crypto library. If a user doesn't want to use a supported library, they can choose another. If a feature provided by some library is insecure, we should obviously avoid that feature in those cases.

The more options and freedom to choose, the better IMO.

[vladimir-v-diaz]

@JustinCappos @lukpueh
I don't recall, but did you recommend that we remove support for multiple crypto backends (just use pyca/cryptograpy) or that we use pycryptodome instead of PyCrypto?

[lukpueh]

As far as I can remember we just agreed to remove pycrypto.

I personally don't see the huge benefit of providing multiple alternatives. Especially since we can only provide it for some of securesystemslib's functionality (rsa keys).

[vladimir-v-diaz]

PyCrypto and multi-library support was removed in #62.