Fix flake8 errors/warnings about import in cubicweb/_exceptions.py
We either ignore errors in case of name re-exports and drop re-exports
from old backwards compatibility. We fix the only occurrence of
NoSelectableObject being imported from cubicweb in tests and update the
import to use lgc instead.
Document: cubicweb-docTitle: CubicWeb DocumentationAuthor: LogilabAbstract: Some base documentation for CubicWeb users and developpersSection: Apps/ProgrammingFormat: HTMLIndex: /usr/share/doc/python-cubicweb-documentation/html/index.htmlFiles: /usr/share/doc/python-cubicweb-documentation/html/*