scripts: release: list_backports.py

Created list_backports.py to examine prs applied to a backport
branch and extract associated issues. This is helpful for
adding to release notes.

The script may also be used to ensure that backported changes
also have one or more associated issues.

Signed-off-by: Christopher Friedt <cfriedt@fb.com>
(cherry picked from commit 57762ca1)