Remove dependency on swh-core.

This is a fix to workaround pip's inability to correctly solve
extra requirements (swh-model depends on swh-core[], but if other
packages depend on swh-model and swh-core[http], the 'http' extra
does not always get installed).

requirements-swh.txt

-swh.core >= 0.0.60

swh/model/cli.py

@@ -9,13 +9,14 @@ import sys
 
 from functools import partial
 
-from swh.core.cli import CONTEXT_SETTINGS
-
 from swh.model import identifiers as pids
 from swh.model.exceptions import ValidationError
 from swh.model.from_disk import Content, Directory
 
 
+CONTEXT_SETTINGS = dict(help_option_names=['-h', '--help'])
+
+
 class PidParamType(click.ParamType):
     name = 'persistent identifier'