scripts: set_assignees.py: Use "size: XS" label for one-liners

Previously, one-line changes were tagged as "Trivial".

The description of the "Trivial" label states:
"Changes that can be reviewed by anyone, i.e. doc changes, minor build
system tweaks, etc.".

Just because a change only affects a single line of code, it does not
mean that it is a trivial change. It may have difficult to understand
implications which require approval of the responsible maintainer.

For this reason, change the label to "size: XS" and let humans judge
if a PR is trivial or not.

Signed-off-by: Martin Jäger <martin@libre.solar>
1 file changed