archival: avoid touching deprecated gzip name attribute
The existing workaround didn't work when no filename was specified.
If running in a context with warnings enabled and subsecond mtime it gave a
warning:
DeprecationWarning: use the name attribute
$ "$TESTDIR/hghave" pyflakes || exit 80
$ cd "`dirname "$TESTDIR"`"
$ pyflakes mercurial hgext 2>&1 | "$TESTDIR/filterpyflakes.py"
hgext/inotify/linux/__init__.py:*: 'from _inotify import *' used; unable to detect undefined names (glob)