Allow using pycryptodome, and prefer it over pycrypto

[mgorny]

pycryptodome is a maintained fork of pycrypto. They are mostly API-compatible (and use the same package name, i.e. meant to be a drop-in replacement), and AFAICS the little bit of API that Beaker uses is fully compatible.

Therefore, list pycryptodome as another extra dependency. Furthermore, since pycrypto is dead and being phased out whenever possible, use pycryptodome for tests instead of it.

[amol-]

Thanks, this is a good idea as pycrypto had some security issues too.