[test] Use our BaseTestCase in unittest_rtags.py
We need it DeprecatedInstanceWithoutModule for assertWarns method. Use this
instead of unittest2 directly with a try/except block to get rid of flake8
error about import not being one top of file.
Document: cubicweb-doc
Title: CubicWeb Documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/python-cubicweb-documentation/html/index.html
Files: /usr/share/doc/python-cubicweb-documentation/html/*