When running rb-site with any modern version of the cryptography

module, a warning will be shown about cryptography dropping Python 2

support. This wasn't caught by the PendingDeprecationWarning, as they

output it as a UserError. We now filter these out, which also catches

a few others that could come up (like pdfminer.six from Power Pack).

Ran rb-site on Python 2.7. Saw the warnings go away.