Adds PulpPluginAppConfig.version warnings
Starting with 3.10 plugins will be required to declare their version as
version attribute on the PulpPluginAppConfig subclass. This adds
warnings starting in 3.9 encouraging plugin writers to do so.
Additionally, it sets the
version on PulpAppConfig so that it doesn't
complain about itself. The bumpversion spec also will bump this string
at release time.