This test is meant to intentionally trigger an exception in the cryptography library
by creating a CMAC with a non-block cipher algorithm, IDEA.
That doesn't work any more because IDEA is now treated as a block cipher algorithm.
To fix this, we now use the ARC4 algorithm instead,
which does trigger the expected exception.

The load_der_public_key method and the load_pem_private_key method were
removed from Backend class in cryptography 42.0.0[1].

Closes #713

[1] pyca/cryptography@41daf2d

... according to the versions actually tested now.

... because now this is not installable in these versions.

This reverts commit 64e4f18
because it broke the CI tests.