v3.21.0 - Bourdeaux

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.