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.
=====================================================|cubicweb| - The Semantic Web is a construction game!=====================================================|cubicweb| is a semantic web application framework, licensed under the LGPL,that empowers developers to efficiently build web applications by reusingcomponents (called `cubes`) and following the well known object-oriented designprinciples.Main Features~~~~~~~~~~~~~* an engine driven by the explicit :ref:`data model <TutosBaseCustomizingTheApplicationDataModel>` of the application,* a query language named :ref:`RQL <RQL>` similar to W3C's SPARQL,* a :ref:`selection+view <TutosBaseCustomizingTheApplicationCustomViews>` mechanism for semi-automatic XHTML/XML/JSON/text generation,* a library of reusable :ref:`components <Cube>` (data model and views) that fulfill common needs,* the power and flexibility of the Python_ programming language,* the reliability of SQL databases, LDAP directories, Subversion and Mercurial for storage backends.Built since 2000 from an R&D effort still continued, supporting 100,000s ofdaily visits at some production sites, |cubicweb| is a proven end to end solutionfor semantic web application development that promotes quality, reusability andefficiency.QuickStart~~~~~~~~~~The impatient developer will move right away to :ref:`SetUpEnv` then to :ref:`ConfigEnv`.Social~~~~~~* Chat on the `jabber forum`_* Discuss on the `mailing-list`_* Discover on the `blog`_* Contribute on the forge_.._Logilab: http://www.logilab.fr/.._forge: http://www.cubicweb.org/project/.._Python: http://www.python.org/.._`jabber forum`: http://www.logilab.org/blogentry/6718.._`mailing-list`: http://lists.cubicweb.org/mailman/listinfo/cubicweb.._blog: http://www.cubicweb.org/blog/1238Narrative Documentation~~~~~~~~~~~~~~~~~~~~~~~A.k.a. "The Book"..toctree:::maxdepth: 2 book/intro/index..toctree:::maxdepth: 2 tutorials/index..toctree:::maxdepth: 3 book/devrepo/index book/devweb/index book/pyramid/index..toctree:::maxdepth: 2 book/admin/index book/additionnal_services/index book/annexes/indexChanges~~~~~~~..toctree:::maxdepth: 2 changes/changelogReference documentation~~~~~~~~~~~~~~~~~~~~~~~API'''..toctree:::maxdepth: 1:glob: api/*..toctree:::maxdepth: 1 book/en/devweb/js_api/indexDeveloppers~~~~~~~~~~~..toctree:::maxdepth: 1:glob: How to contribute to the code base <https://hg.logilab.org/master/cubicweb/file/tip/README#l39> General contribution guide for cubes <https://www.logilab.org/Card/contributing> dev/*Indexes~~~~~~~* the :ref:`genindex`,* the :ref:`modindex`,