Fix #826: Suppress cryptography FFDH deprecation warning and add decrepit fallback

[sarthak-shubham]

Fixes #826

Addresses the CryptographyDeprecationWarning raised when using FFDH key exchange with cryptography v50.0.0+.

Changes:

Wraps FFDH operations in warnings.catch_warnings() to suppress CryptographyDeprecationWarning locally, preventing it from leaking into end-user logs.
Adds a try/except ImportError fallback to import dh from cryptography.hazmat.decrepit.asymmetric if it is eventually removed from primitives, following the same pattern used for deprecated symmetric ciphers in crypto/cipher.py.
Tested with python -m unittest tests.test_kex — all 14 tests pass.

[sarthak-shubham]

@ronf following up from #826 - I went with a try/except ImportError fallback scoped to the single dh symbol here, rather than the _algs/_decrepit_algs registry from cipher.py, since there's only one thing to fall back on here, not a list. Let me know if you'd rather I match the registry pattern for consistency.

Also, the CI workflow is waiting on approval to run whenever you get a chance.

[ronf]

Is there a way to do this which avoids having to insert the warnings block in each of the DH class methods? I see that the decrepit module doesn't yet have the asymmetric algorithms in it, so we can't count on that succeeding to avoid the warnings. However, I'd really like to find a way to suppress the warnings once at import time and not have to go through that code every time a DH key is instantiated.