BUG: ElasticNectCV choosing improper l1_ratio

The code was lacking good tests: it had only smoke tests. Shame on
me (I am the author).
PY3 allow multiple base classes in six.with_metaclass

Imported from https://bitbucket.org/gutworth/six/pull-request/10/fix-for-23-by-allowing-to-specify-multiple/diff
DOC correct / simplify dbscan examle