[hoangchunghien]

Closes: #16561

stubtest failed on the stubsabot bump with one error: paramiko.kex_mlkem failed to find stubs. paramiko 5.1 adds that module for the mlkem768x25519-sha256 hybrid key exchange (ML-KEM-768 with X25519), and Transport prefers it when it is available.

The new kex_mlkem.pyi follows kex_curve25519.pyi: the two message constants, and KexMLKEM768X25519 with name, hash_algo, transport, the two private keys, is_available, start_kex and parse_next.

mlkem_key holds a cryptography.hazmat.primitives.asymmetric.mlkem.MLKEM768PrivateKey, which only exists from cryptography 48, while these stubs require cryptography>=37.0.0. Rather than raise that floor, I typed it as Incomplete | None with a comment. Happy to switch to the real type and bump the dependency if you prefer.

stubtest for paramiko 5.1, mypy_test.py stubs/paramiko, pyright_test.py stubs/paramiko (with cryptography installed), black, ruff and flake8 pass locally.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉