Log Message:
py-cryptodome: update to 3.21.0.

3.21.0 (30 September 2024)
++++++++++++++++++++++++++

New features
---------------
* By setting the PYCRYPTODOME_DISABLE_GMP environment variable,
  the GMP library will not be used even if detected.
* Add support for Curve25519 / X25519.
* Add support for Curve448 / X448.
* Add attribute ``curve`` to EccPoint and EccXPoint classes,
  with the canonical name of the curve.
* GH#781: the label for the SP800_108_Counter KDF may now
  contain zero bytes. Thanks to Julien Rische.
* GH#814: RSA keys for PSS can be imported.

Resolved issues
---------------
* GH#810: fixed negation of Ed25519 points.
* GH#819: accept an RFC5916 ECPrivateKey even if it doesn't
  contain any of the optional elements
  (parameters [0] and publicKey[1]).

Other changes
-------------
* Remove support for Python 3.5.