Fix UUID json serialization

[eric-eclecticiq]

Fix #243

UUID json serialization

Solution

The problem is that by default the JSON encoder can not transform UUID class to JSON. I introduced UuidJSONEncoder that will invoke str on it when making the HTTP answer.

Testing issue

The tests did not catch the issue as it was mocked already using string UUID. I updated the mocked fixtures to be UUID as when coming from the DB.

Types

1. The TAXII2 models (DB) were already correctly relying on UUID. However, opentaxii.taxii2.entities documented str types. Instead, I updated to UUID when the TAXII2 specs expect an identifier
2. OpenTAXII2PersistenceAPI method interfaces also highlight when a UUID is expected
3. Transform api_root_id and job_id from opentaxii.server.TAXII2Server URL path to UUID. It makes less divergence with UUID used elsewhere. Only the path parameter collection_id_or_alias can not be converted to UUID as it could also be an alias that is a string.

Technical improvements

Typing

1. Add mypy and some stubs as dependency
2. Add mypy check in the CI
3. Enforce typing check on opentaxii/ and tests although some files are excluded as there were too many issues to fix

Test package

tests/ was not a package and used pytest magic to find root modules for tests. It started to create a conflict for mypy as there were 2 utils modules. I added a __init__.py where missing and updating utils and fixtures imports.

[saaj]

Unifying to UUIDs is a good change.