Tx-XMPP 0.12 has beaking change in `discoFeature.identities` because it now supports
`xml:lang` attribute, so this update implies updating the code everywhere were this is
used.
XEP-0420 bounds starts at 0 but 0-length rpad defeats its purpose (the XEP actually says
"TODO: sane boundaries?"). Furthermore, some tests where failing randomly due to 0-lenght
rpad sometimes happening. This patch makes rpad minimum size at 50.
- split the plugin in several classes, with models, exceptions, and backends separated.
- add a backend for `python-gnupg`, as it can be pip installed without having to use
`--system-site-packages` in the virtual environment.
- use modern syntax for type hints.
rel 475
