openapi: make the code python 3.5 compatible
openapi: make the code python 3.5 compatible

It is common to use Ubuntu 16.04 to build snaps. For example,
the official docker container to build snaps is using this old
distribution.

However, Ubuntu 16.04 ships Python 3.5.X which is not compatible
with the f-strings in generate_openapi.py. This is sad, because
we need to use the `.format()` syntax to make it compatible.

wekan/wekan,GhassenRjab/wekan,wekan/wekan,GhassenRjab/wekan,libreboard/libreboard,wekan/wekan,wekan/wekan,wekan/wekan,GhassenRjab/wekan,libreboard/libreboard