Fix Spatz python dependencies in Carfield top level

[ezelioli]

Spatz hw generation python scripts rely on two additional dependencies (jsonref, jsonschema) not listed in requirements.txt.

Moreover, the Spatz makefile will default to python3.6 if the PYTHON variable is not set. Therefore, changed it in carfield.mk to default to python3.

Finally, remove the duplicated hjson dependency.

[alex96295]

LGTM, before merging, can you please edit the docs with Python dependency requirement? We can have >= 3.0

[bluewww]

I don't thik python 3.0 is valid dependency as most of the libraries won't work at that version. It should be put to 3.6 which is the version used on alma linux 8

[yvantor]

@ezelioli @alex96295 I wanted just to point out that with #245 we will be no longer using the generated Spatz wrapper, replaced by this

[alex96295]

@ezelioli @alex96295 I wanted just to point out that with #245 we will be no longer using the generated Spatz wrapper, replaced by this

We'll merge this since it's a fix and then incrementally add the other changes