matrix-synapse: 1.7.1 -> 1.7.2

https://github.com/matrix-org/synapse/releases/tag/1.7.2

Building for now with Python 3.7 as Python 3.8.1 changed it's behavior
regarding URL parsing[1] which broke one of its dependencies[2].

[1] https://bugs.python.org/issue27657
[2] https://hydra.nixos.org/build/108652790 / https://github.com/mozilla/bleach/issues/503