security/py-keyrings.alt: adjust optional dependency (unbreak)

In commit a16d8f14ce3b, deprecation of `security/py-pycrypto' had
not been handled completely: one optional dependency which is off
by default was still referencing the expiring port.

According to release notes, EncryptedFileKeyring now supports both
pycryptodome and pycryptodomex (preferring the latter), so prefer
the latter.

Reported by: someone on t.me/freebsd_ru