[devtools] Use XMLValidator instead deprecated SaxOnlyValidator
The renaming was introduced in 198fdadafed6.o
This prevent a meta class conflict introduced by logilab-common 0fcd2e050621.
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/devtools/ usr/lib/PY_VERSION/site-packages/cubicweb/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/skeleton/ usr/lib/PY_VERSION/site-packages/cubicweb/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/test usr/lib/PY_VERSION/site-packages/cubicweb/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/entities/test usr/lib/PY_VERSION/site-packages/cubicweb/entities/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/ext/test usr/lib/PY_VERSION/site-packages/cubicweb/ext/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/server/test usr/lib/PY_VERSION/site-packages/cubicweb/server/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/sobjects/test usr/lib/PY_VERSION/site-packages/cubicweb/sobjects/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/hooks/test usr/lib/PY_VERSION/site-packages/cubicweb/sobjects/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/web/test usr/lib/PY_VERSION/site-packages/cubicweb/web/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/etwist/test usr/lib/PY_VERSION/site-packages/cubicweb/etwist/