The change from https://reviews.reviewboard.org/r/7111/ resulted in
PyCrypto being required for all deployments of ReviewBoard.
Previously, it would have only been required if certain SCMs were in
use (beanstalk, bitbucket, gitlab or unfuddle).

However, it is now imported and used in the server initialization,
which means we need to ensure that it is on the system.

Yes, most systems will have pycrypto available already, but it's
always better to be explicit and avoid unexpected failures.