net-im/py-matrix-synapse: Update to 1.12.4

Changelog: https://github.com/matrix-org/synapse/blob/v1.12.4/CHANGES.md

PR: 246150
Submitted by: shibayashi@cypherpunk.observer
Approved by: Sascha Biberhofer <ports@skyforge.at> (maintainer)
MFH: 2020Q2 (blanket: bugfix release)
X-MFH-with: 533398