Fix Python 3.13 telnetlib import - #105

Summary

• make the telnetlib import in the Telnet scanner optional so missing stdlib telnetlib does not break module import at startup
• add standard-telnetlib for Python 3.13+ installs so the stdlib-compatible telnetlib module is available from requirements
• report a clear Telnet scanner error if the scanner is used without a telnetlib provider installed

Fixes #104.

Validation

• Not run locally
• git diff --check
• rg -n "import telnetlib|from telnetlib|telnetlib\\.|standard-telnetlib" changeme requirements.txt