While here,
* Add OTP HID support for FreeBSD (merged in upstream) [0];
* Move into pep517 (https://wiki.freebsd.org/Python/PEP-517);
* Start using USES=pycryptography as introduced by 7bb64b89d0e5ec8

Note that,
* should one does not with to set PYCRYPTOGRAHY_DEFAULT=legacy,
  `ykman` (and other packages/ports depending on that) would require
  CRYPTOGRAPHY_OPENSSL_NO_LEGACY= to be true.

[0] https://github.com/Yubico/yubikey-manager/commit/ecd7897b3f020542f70581f77f47ba57c739b334