Yep.  If a software updater integrating TUF wants to also support RSSA-PSS signatures, they will require clients to install `pip install tuf[tools]`.  I suppose an additional installation option (i.e., `pip install tuf[rssa-pss]`, which will install PyCrypto) can be added, but I wanted to keep it simple at two options.

I also wanted to improve usability by having a pure Python default and avoiding the issues that come with compiling C extensions.  Verifying ed25519 signatures with the optimized pure Python module no longer suffers from speed issues.