Filter out warnings caused by cryptography in rb-site.
Filter out warnings caused by cryptography in rb-site.

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).

Testing Done:
Ran `rb-site` on Python 2.7. Saw the warnings go away.

Reviewed at https://reviews.reviewboard.org/r/11360/

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