upload: use f-string

Change-Id: I91b99a7147c7c3cb5485d5406316c8ffd79f9272
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/374914
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Joanna Wang <jojwang@google.com>
Tested-by: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
diff --git a/subcmds/upload.py b/subcmds/upload.py
index f9c5e7f..8d949be 100644
--- a/subcmds/upload.py
+++ b/subcmds/upload.py
@@ -657,7 +657,7 @@
 
                 branch.uploaded = True
             except UploadError as e:
-                self.git_event_log.ErrorEvent("upload error: " + str(e))
+                self.git_event_log.ErrorEvent(f"upload error: {e}")
                 branch.error = e
                 branch.uploaded = False
                 have_errors = True