security/py-vici: Update to 5.9.3

Decode error messages in Python bindings. Otherwise one might end up
with b'<errmsg>' in the output.

Changelog: https://github.com/strongswan/strongswan/releases/tag/5.9.3

PR: 258410
MFH: 2021Q3
(cherry picked from commit 41fe31b3e8679c83fb31575a973f18328fe41d49)