[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.
usr/lib/python2*/*-packages/cubicweb/dataimport/
usr/lib/python2*/*-packages/cubicweb/server/
usr/lib/python2*/*-packages/cubicweb/hooks/
usr/lib/python2*/*-packages/cubicweb/sobjects/
usr/lib/python2*/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/