LU-15655 contrib: update branch_comm to python3

LU-15655 contrib: update branch_comm to python3

Update the branch_comm script from to python2 to python3.

Lustre-change: https://review.whamcloud.com/46856
Lustre-commit: c6b257254878387976247fea29bd2b142cd03e5e

Test-Parameters: trivial
Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Change-Id: Icbb747c01e15cbe77c6dd6df3e913654ded84239
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/53947
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>