scripts: fix py3.6 compat in update_default_servers.py

Note: this was causing a weird flake8 error on CI. Presumably due to CI running flake8 via py3.7.

```
flake8 . --count --select=$ELECTRUM_LINTERS --show-source --statistics
./electrum/scripts/update_default_servers.py:1:26: E999 SyntaxError: invalid syntax
#!/usr/bin/env python3

^
1     E999 SyntaxError: invalid syntax
1
```

electrum/scripts/update_default_servers.py

@@ -56,7 +56,7 @@ async def f():
 
         print(json.dumps(res_servers, indent=4, sort_keys=True))
         print(f"got reply from {len(old_servers_online)}/{len(old_servers_all)} old servers", file=sys.stderr)
-        print(f"{len(newly_added_servers)=}. total: {len(res_servers)=}", file=sys.stderr)
+        print(f"len(newly_added_servers)={len(newly_added_servers)}. total: {len(res_servers)}", file=sys.stderr)
     finally:
         stopping_fut.set_result(1)