[web/request] don't play tricks with utc offset
We need an UTC datetime from a time.time()-type value. It turns out
datetime provides a method to do just that. Related to #2154655.
usr/lib/PY_VERSION/*-packages/cubicweb/entities/
usr/lib/PY_VERSION/*-packages/cubicweb/ext/
usr/share/cubicweb/cubes/
usr/lib/PY_VERSION/*-packages/cubicweb/*.py