Revert "Remove stretch from uprev script"

This reverts commit fb64d2fd1656e93cc60be8652a1079dcf74f0666.

Reason for revert: Per discussion, we are not yet ready to get rid of stretch

Original change's description:
> Remove stretch from uprev script
>
> We are dropping test support for stretch, as our users now have an
> upgrade path.
>
> BUG=chromium:1154444
> TEST=none
>
> Disallow-Recycled-Builds: test-failures
> Change-Id: I4c2f6e4a5d5276d55f6a1d7e354d9b91efbd87a3
> Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/containers/cros-container-guest-tools/+/2570834
> Reviewed-by: David Munro <davidmunro@google.com>
> Tested-by: kokoro <noreply+kokoro@google.com>
> Commit-Queue: Fergus Dall <sidereal@google.com>

Bug: chromium:1154444
Change-Id: I04353c07b31cbced277b0c7123de6f1342663fed
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/containers/cros-container-guest-tools/+/2583422
Reviewed-by: Fergus Dall <sidereal@google.com>
Reviewed-by: David Munro <davidmunro@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Commit-Queue: Fergus Dall <sidereal@google.com>
diff --git a/scripts/uprev.py b/scripts/uprev.py
index 6c326cd..2ab9e22 100755
--- a/scripts/uprev.py
+++ b/scripts/uprev.py
@@ -55,7 +55,7 @@
             file_arch = arch
 
         for ctype in ['test', 'app_test']:
-            for release in ['buster']:
+            for release in ['stretch', 'buster']:
                 base_url = f'gs://cros-containers-staging/{milestone}' \
                     f'/images/debian/{release}/{arch}/{ctype}/{latest_container}/'