How to configure Geany to allow Format > Comment in types Geany do not recognize?
There is no way of configuring comments outside a filetype, you would need to define a custom filetype. But in the case of `.pyi` Geany can be configured to recognise that extension as Python filetype in the way @neoh4x0r mentioned using `filetype_extensions.conf`
If somebody wants to make a PR adding the `.pyi` extension it probably would be accepted.