net-im/py-matrix-synapse: Update to 1.12.3, Fixes security vulnerability

Changelog: https://github.com/matrix-org/synapse/blob/v1.12.3/CHANGES.md

PR: 244279
Submitted by: Sascha Biberhofer <ports@skyforge.at> (maintainer)
Reported by: Alexander Sieg <ports@xanderio.de>
MFH: 2020Q2
X-MFH-with: 532268
Security: 1afe9552-5ee3-11ea-9b6d-901b0e934d69