Use convertkey.py from the python-potr package

We need Debian bug #780610 resolved. Then rewrite eb4abc9 in test/9062-otr-bot-with-python-potr appropriately.

Feature Branch: test/9066-use-convertkey-from-Debian

Original created by @anonym on 9066 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information