[web] Stop using negative indices with os.stat results
The python doc says:
More items may be added at the end by some implementations.
.st_mtime is also more readable than [-2].
usr/lib/PY_VERSION/*-packages/cubicweb/web
usr/lib/PY_VERSION/*-packages/cubicweb/wsgi
usr/share/cubicweb/cubes/shared/data
usr/share/cubicweb/cubes/shared/wdoc