remove member property of BackendModule and func_returns

and change both into __root__ models

as discussed in #36 (closed), this MR changes the expected and the returned requests. Both are now so-called root models with pydantic. I.e. they except the arguments directly.

• Changes to the input for a consumer can be seen in one of the test requests, e.g. here
• changes to the returned values can be seen here

Note that in pydantic, we need to use the __root__ property to access the real data. But it does not require __root__ to be in the parsed JSON that is parsed to parse_raw or parse_obj (see the docs).

Final note: This MR does not yet implement the possibility to display the return schema for a model. I'd like to do this when I work on #19 (closed)

@daniel-eggert: it would be nice if you could have a look at it

closes #36 (closed)