[html generation] even when doctype without extension are asked, we should define the xmlns:cubicweb attribute to avoid html validation error during tests (break tests with new calendar view in default branch since it demote to html)

# copyright %(year)s %(author)s, all rights reserved.
# contact %(author-web-site)s -- mailto:%(author-email)s
#
%(long-license)s
"""%(distname)s entity's classes"""