Add default keyword values for main IIIF objects

[giacomomarchioro]

This pull request add the default keywords to the JSON shema for the main IIIF object classes.

E.g. the default keyword for Manifest type is "Manifest".

The default keywords are useful for solving iiif-prezi/iiif-prezi3#12.

[glenrobson]

Can we drop 3.6 in the test and add 3.10 which is the latest python version.

[glenrobson]

Glen to create a new pull request and let Giacomo know when its ready.

[glenrobson]

I've just been looking at something and came across a const keyword:

https://json-schema.org/understanding-json-schema/reference/generic.html#constant-values

Would this be more suitable than default?

[giacomomarchioro]

I've just been looking at something and came across a const keyword:

https://json-schema.org/understanding-json-schema/reference/generic.html#constant-values

Would this be more suitable than default?

Yes, it makes much more sense from the semantic point of view, however, all my attempts using datamodel-codegen failed, maybe it is linked with this issue: koxudaxi/datamodel-code-generator#658

[glenrobson]

Ah thats a shame. Oh well maybe we can change it to const when its supported.

I've removed python 3.6 from the main branch so if you update the tests should work then and I can merge.

[glenrobson]

git checkout main
git pull origin 
git checkout defaulttypevalue
git merge main
git push

[coveralls]

Coverage remained the same at 78.231% when pulling 0da122b on giacomomarchioro:defaulttypevalue into 1590fae on IIIF:main.