Unignore and enforce check E731 ("Do not assign a lambda expression, use a def")

[ShadowJonathan]

(E731 checks "Do not assign a lambda expression, use a def")

This check is voilated in only a few cases;

synapse/util/caches/descriptors.py:500:5: E731 do not assign a lambda expression, use a def
synapse/util/caches/descriptors.py:542:5: E731 do not assign a lambda expression, use a def
tests/server.py:332:9: E731 do not assign a lambda expression, use a def

The first to related to caching and then wrapping the callback functions in lambdas, and the last one being ThreadedMemoryReactorClock.callFromThread, I think it's better to remove this violation from the ignore list, and then to add # noqa: E731 statements on those specific places.