* Don't use a virtualenv

* Generate the server's signing key to allow it to start

* Add signing key paths to CI configuration files

* Use a Python script to create the postgresql database

* Improve logging

This adds:

* a test sqlite database
* a configuration file for the sqlite database
* a configuration file for a postgresql database (using the credentials in `.buildkite/docker-compose.pyXX.pgXX.yaml`)

as well as a new script named `.buildkite/scripts/test_synapse_port_db.sh` that:

1. installs Synapse
2. updates the test sqlite database to the latest schema and runs background updates on it
3. creates an empty postgresql database
4. run the `synapse_port_db` script to migrate the test sqlite database to the empty postgresql database (with coverage)

Step `2` is done via a new script located at `scripts-dev/update_database`.

The test sqlite database is extracted from a SyTest run, so that it can be considered as an actual homeserver's database with actual data in it.

* Delete format_tap.py

This python implementation of a tap formatting library for buildkite has been
replaced with a perl implementation as part of the matrix-org/sytest repo,
which is specific to sytest's language, not that of any one homeserver's.

This also adds a worker blacklist.

* Add a sytest blacklist file

* Add changelog

* Add blacklist to manifest

Including workers!

* remove 2.7 from CI and publishing

* fill out classifiers and also make it not be installed on 3.5

* some minor bumps so that the old deps work on python 3.5

Sometimes the build agents get lost or die (error codes -1 and 2). Retry automatically a maximum of 2 times if this happens.

Error code reference:

* -1: Agent was lost
* 0: Build successful
* 1: There was an error in your code
* 2: The build stopped abruptly
* 255: The build was cancelled

* Regen sample config before kicking off agents

* Add changelog