Add missing dependency on attr

We've rcently added a dep on `attr`. I don't know why the CI didn't pick this
up, but we should make it explicit anyway.

synapse/python_dependencies.py

@@ -57,6 +57,7 @@ REQUIREMENTS = {
     "phonenumbers>=8.2.0": ["phonenumbers"],
     "six": ["six"],
     "prometheus_client": ["prometheus_client"],
+    "attr": ["attr"],
 }
 
 CONDITIONAL_REQUIREMENTS = {