I know almost nothing about the ethiopic calendar, but maybe I can help with some starting points and ideas.
You should probably use the gregorian based datetime
objects internally and convert from and to the ethiopic calendar before displaying or after receiving input from the user. This is assuming all dates in the ethiopic calendar can be represented in the gregorian and vice versa.
You can maybe use a package from PyPI like this: ethiopian-date-converter
If you really need to use ethiopic dates internally in your code, you would probably need to roll your own datetime-like object, ModelField, FormField etc. Storing ethiopic dates in a database is probably also an adventurous task. It might not be worth it, depending on the task you are trying to solve.
If you can get away with gregorian dates internally (in the database, in your algorithms etc.), you might only need to build a template filter for output and a custom Field for input.
I hope this will get you started.