Python 3.13 support: vendor telnetlib, paramiko 3.x — cloudshell-cli 6.0

[nahumtimerman]

Why

cloudshell-cli 5.0.2 cannot run on Python 3.13:

1. telnetlib was removed from the stdlib in Python 3.13 (PEP 594) — TelnetSession fails at import.
2. paramiko~=2.12 is EOL-ish and creates friction with modern cryptography (the ecosystem now ships cryptography>=43 via cloudshell-snmp 6.0).

What changed

• Vendored telnetlib: CPython 3.12's Lib/telnetlib.py copied to cloudshell/cli/session/_telnetlib.py (PSF-2.0, provenance header, CLI test()/__main__ scaffolding and the 3.13 deprecation call removed). telnet_session.py does try: import telnetlib / except ImportError: ... _telnetlib as telnetlib — the stdlib module is still preferred on <=3.12, so behavior there is unchanged. No other module in the repo imports telnetlib.
• paramiko >=3.4,<4 (resolves to 3.5.1). Capped below 4.x deliberately: SSHSession._get_pkey_object still supports paramiko.DSSKey, which paramiko 4.0 removed; the _transport_factory server_extensions ssh-rsa workaround is already hasattr-guarded. scp~=0.14 left as-is (resolves to 0.16.1, compatible).
• Version 6.0.0 (major: dependency major + vendored module), python_requires>=3.9, classifiers 3.9–3.13.
• CI: package-tox-py-37-39.yml → package-tox-py-39.yml; tox envlist py39 only.
• Lint: setuptools<81 pinned into the flake8 hook (its plugins need pkg_resources, gone from newer setuptools/py3.12+ venvs); vendored module excluded from hooks/flake8; per-file-ignores for E231/E702 false positives from the py3.12+ f-string tokenizer in tl1_session.py and A005 for the intentional cloudshell/cli/types.py module name.

Evidence

• Tests: full suite 130/130 passed on Python 3.13.x and 3.9.0 (fresh venvs, paramiko 3.5.1). The SSH tests spin up a real in-process paramiko Transport server (password + RSA-key auth, SCP and SFTP upload) — a live handshake against paramiko 3.x. No mock changes were needed.
• Telnet smoke (py3.13): import telnetlib confirmed to fail on 3.13; a local asyncio telnet server on port 2323 + TelnetSession.connect() completed the full login/password action-map flow and prompt match through the vendored module (telnetlib module in use: cloudshell.cli.session._telnetlib, CONNECTED, active = True).
• SSH smoke (py3.13): paramiko 3.5.1 import + SSHSession instantiation OK (plus the live-handshake unit tests above).
• pre-commit: pyupgrade, isort, black, flake8 all pass.

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