python: Convert "".format to Python 3.6 f-strings.

Generated by pyupgrade --py36-plus --keep-percent-format, but with the
NamedTuple changes reverted (see commit
ba7906a3c6, #15132).

Signed-off-by: Anders Kaseorg <anders@zulip.com>

scripts/lib/sharding.py

@@ -47,7 +47,7 @@ with open('/etc/zulip/nginx_sharding.conf.tmp', 'w') as nginx_sharding_conf_f, \
             if '.' in shard:
                 host = shard
             else:
-                host = "{}.{}".format(shard, external_host)
+                host = f"{shard}.{external_host}"
             assert host not in shard_map, "host %s duplicated" % (host,)
             shard_map[host] = int(port)
             write_realm_nginx_config_line(nginx_sharding_conf_f, host, port)