iotests: use with-statement for open() calls

iotests: use with-statement for open() calls

Silences a new pylint warning. The dangers of *not* doing this are
somewhat unclear; I believe the file object gets garbage collected
eventually, but possibly the way in which it happens is
non-deterministic. Maybe this is a valid warning, but if there are
consequences of not doing it, I am not aware of them at present.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20210720173336.1876937-2-jsnow@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>