External plugins need to know which plugin interface is being
used by the host OSC. Releases <1.6 (?) will not have a
version defined.
Plugins can add the following to their make_client() to discover
the interface version:
_plugin_interface_version = getattr(
instance,
"PLUGIN_INTERFACE_VERSION",
None,
)
Change-Id: Ifc0f40fec5bc27f6425139984936b7f6e032a580