[FEATURE] tools/flash_writer.py depends on deprecated telnetlib.

### Is your feature request related to a problem? Please describe.

This tool will not work on Python 3.13+, welcome to the brave new world of constant changes ;-)

```
+ /usr/bin/time -h ./tools/flash_writer.py -s -c /dev/cuaU0 -d -b 115200 nuttx.spk
/tmp/nuttx/nuttx.git/./tools/flash_writer.py:27: DeprecationWarning: 'telnetlib' is deprecated and slated for removal in Python 3.13
import telnetlib
```

### Describe the solution you'd like

telnetlib probably needs to be replaced/updated to something that will provide backward/forward comatiblity maybe conditional include :-)

### Describe alternatives you've considered

I am using only serial protocol so telnet may not be even included here :-)

### Verification

- [x] I have verified before submitting the report.