...  to allow installation alongside our matrix-synapse transitional package.

Make sure we don't put the _trial_temp directory in the package target
directory.

Fixes https://github.com/matrix-org/synapse/issues/4322

since #4298, the optional dependencies are no longer installed with a simple
`pip install .`, which meant that they were not being included in the debian
package.

The easy fix to that is dh_virtualenv --extras, but that needs dh_virtualenv
1.1...

Better follow our own release notes.