problems with _apt user privileges in upgrading from Jessie to Stretch [solved?]

Posting here in case this might help others who may be encountering the same problem.

I really appreciate the enhanced security provided for apt in the new release. But one of the changes caused me a small headache in upgrading.
Following the upgrade, running `apt get update' resulted in this warning:

Reading package lists... Done
W: Download is performed unsandboxed as root as file '/var/lib/apt/lists/partial/deb.debian.org_debian_dists_stretch_InRelease' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)

which meant, I suppose, that I wasn't getting all the benefits of the new regime. After a bit of reading and a lot of trial and error I was able, I think, to resolve the issue by manually changing the owner attribute of the directory /var/lib/apt/lists/:

chown -R _apt.root lists

(executed in /var/lib/apt/ )

It doesn't seem to be enough to have /var/lib/apt/lists set to rwxr_xr_x if it's owned by root.

If anyone has a different or better solution, I'd be curious to hear about it,

Jim