CryptographyDeprecationWarning: CFB warning fires on import even when AESCipher isn't used #1033

Every time saml2.cryptography.symmetric is imported, cryptography 44+ emits:

saml2/cryptography/symmetric.py:108: CryptographyDeprecationWarning: CFB has been moved to cryptography.hazmat.decrepit.ciphers.modes.CFB and will be removed from cryptography.hazmat.primitives.ciphers.modes in 49.0.0.
  "cfb": _ciphers.modes.CFB,

This comes from the class-level POSTFIX_MODE dict on the already-deprecated AESCipher class, which is built unconditionally at import time, even if AESCipher itself is never used.

A lazy lookup (build the dict inside build_cipher, or resolve CFB conditionally based on the installed cryptography version, since hazmat.decrepit only exists since cryptography 43) would avoid the warning for anyone who isn't using AESCipher.

(pysaml2 7.5.4, cryptography 49.0.0)