scripts: release: list_backports: use older python dict merge method

In Python versions >= 3.9, dicts can be merged with the `|` operator.

This is not the case for python versions < 3.9, and the simplest way
is to use `dict_c = {**dict_a, **dict_b}`.

Signed-off-by: Christopher Friedt <[email protected]>
(cherry picked from commit 3783cf8)

Author: Christopher Friedt

scripts/release/list_backports.py

@@ -254,7 +254,8 @@ def get_issues(self):
                 self._pulls_without_an_issue.append(p)
                 continue
 
-            issue_map = issue_map | issues_for_this_pr
+            # FIXME: when we have upgrade to python3.9+, use "issue_map | issues_for_this_pr"
+            issue_map = {**issue_map, **issues_for_this_pr}
 
         issues = list(issue_map.values())