Add comment as to why we're pinning black in tests

tox.ini

@@ -114,7 +114,7 @@ skip_install = True
 basepython = python3.6
 deps =
     flake8
-    black==19.3b0
+    black==19.3b0  # We pin so that our tests don't start failing on new releases of black.
 commands =
     python -m black --check --diff .
     /bin/sh -c "flake8 synapse tests scripts scripts-dev scripts/hash_password scripts/register_new_matrix_user scripts/synapse_port_db synctl {env:PEP8SUFFIX:}"