This allows trailing commas in multi-line arg lists.

Minor, but we might as well keep our formatting current with regard to
our minimum supported Python version.

Signed-off-by: Dan Callahan <danc@element.io>

The CI appears to use the latest version of isort, which is a problem when isort gets a major version bump. Rather than try to pin the version, I've done the necessary to make isort5 happy with synapse.

This caused `prev_state_ids` to be incorrect if the state event was not
replacing an existing state entry.

* Factor out functions for injecting events into database

I want to add some more flexibility to the tools for injecting events into the
database, and I don't want to clutter up HomeserverTestCase with them, so let's
factor them out to a new file.

* Rework TestReplicationDataHandler

This wasn't very easy to work with: the mock wrapping was largely superfluous,
and it's useful to be able to inspect the received rows, and clear out the
received list.

* Fix AssertionErrors being thrown by EventsStream

Part of the problem was that there was an off-by-one error in the assertion,
but also the limit logic was too simple. Fix it all up and add some tests.

I got fed up with always adding '@unittest.DEBUG' every time I needed to debug a test.