backup: flush stderr after all writes

backup.py

@@ -122,6 +122,7 @@ class Backup:
         c = colors[letter] if letter in colors else 0
         b = "" if bold else "\033[22m"
         sys.stderr.write(f"\033[1;{c}m{letter}:{b} {msg}\033[0m\n")
+        sys.stderr.flush()
         self.logs.append((letter, msg))
 
     def run(self, outfile: typing.IO[bytes]):
@@ -382,6 +383,7 @@ def main(argv: typing.List[str]):
             backup.run(borg.stdin)
         except BrokenPipeError:
             sys.stderr.write(f"broken pipe\n")
+            sys.stderr.flush()
         finally:
             try:
                 borg.stdin.close()