refactor(mypy): Improve HSMixin type hints

Inside HSMixin:
- when type checking, define `get_type` within HSMixin
- remove now useless type:ignore comment inside `get_description` method

Inside nodes.Constant the get_type could return a str or None:
- make sure `get_type` always returns a str
- change the default value of _uidtype from None to empty string
- type self.uidtype as a str
7 jobs for !49 with topic/default/experiment in 3 minutes and 45 seconds (queued for 35 seconds)
latest detached
Status Name Job ID Coverage
  Tests
passed black #357823

00:00:33

passed check-manifest #357825

00:00:34

passed flake8 #357822

00:00:31

passed mypy #357824

00:00:42

passed py3 #357820

00:00:33

passed py3-from-forge #357821

00:00:49

 
  After Tests
failed trigger-cubicweb-pipeline