[pkg] Use io.open with utf-8 encoding to read long description from README This forces decoding so that this is consistent between Python 2 and 3 as, in Python 3, decoding which would occur anyways and may fail depending on the environment locale settings.

missing-dep-for-interpreter make => make | build-essential | dpkg-dev (usr/*/cubicweb/skeleton/debian/rules.tmpl)