security/vuxml: New entry for sysutils/py-salt vulnerabilities

There are three security vulnerabilities described for sysutils/py-salt
in version 3002[1]: CVE-2020-16846, CVE-2020-17490, and VE-2020-25592.

[1] https://docs.saltstack.com/en/latest/topics/releases/3002.1.html

It is planned to update the port sysutils/py-salt soon, see PR 251013

Reported by: michael.glaus@hostpoint.ch (in PR 251013)
Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D27189