net-im/py-slixmpp: Update to 1.4.2

This release contains a fix for CVE-2019-1000021.

Changelog:
https://lab.louiz.org/poezio/slixmpp/tags/slix-1.4.2

Reviewed by: krion
Approved by: krion (mentor)
MFH: 2019Q1
Security: 526d9642-3ae7-11e9-a669-8c164582fbac
Security: CVE-2019-1000021
Differential Revision: https://reviews.freebsd.org/D19397