net-im/py-matrix-synapse: Update to 1.21.2, Fix security issue

Changelog: https://github.com/matrix-org/synapse/blob/v1.21.2/CHANGES.md

PR: 249948
Submitted by: Sascha Biberhofer <ports@skyforge.at> (maintainer)
MFH: 2020Q4
Security: 5f39d80f-107c-11eb-8b47-641c67a117d8