Personally I would suggest not adding manually because:
1) manual additions are likely to be forgotten when the script @eht16 mentioned gets run
2) since Geany does not know which Python is being used it will highlight async and friends in old code where they are not keywords. Better to wait until they are keywords in at least some code by which time many non-keyword uses will have been fixed.
But you can add them yourself you know, `Menu->Tools->Configuration files->Filetype configuration->scripting languages->filetypes.python` will open the appropriate file and save it to your user configuration directory. Just add them to the `primary=` line.