[AIRFLOW-505] Support unicode characters in authors' names

Closes #1792 from jlowin/pr-tool-ascii

dev/airflow-pr

@@ -308,7 +308,7 @@ def merge_pr(pr_num, target_ref, title, body, pr_repo_desc, local):
             primary_author = distinct_authors[0]
 
         authors = "\n".join(["Author: %s" % a for a in distinct_authors])
-        merge_message_flags.append('--author="{}"'.format(primary_author))
+        merge_message_flags.append(u'--author="{}"'.format(primary_author))
 
     else:
         # This will mark the PR as merged