sync: do not require python 3.9

Use pre-3.9 syntax for NamedTuple, so that users do not need to have
python 3.9 or later installed.

Bug: b/255632143, crbug.com/gerrit/16355
Test: manually verified with python 3.8
Change-Id: I488d2d5267ed98d5c55c233cc789e629f1911c9d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/349395


Tested-by: LaMont Jones <lamontjones@google.com>
Reviewed-by: Jonathan Nieder <jrn@google.com>