net-im/py-matrix-synapse: Update to 1.57.1

• Move py-idna dependency to TEST_DEPENDS
• Add a dependency for Python < 3.8

PR: 263430
Approved by: Sascha Biberhofer <ports@skyforge.at> (maintainer)

(cherry picked from commit 45f1b936769d8038de890dda01ce7986a94bbbe6)