After https://github.com/matrix-org/synapse/pull/8377, the deb packages no longer indirectly installed the `"test"` dependencies, causing debian packages to fail to build while carrying out the unit tests.
This PR installs `test` dependencies explicitly when building debian packages.
In the debian package, make the virtualenv symlink python to /usr/bin/python3.X
rather than /usr/bin/python3. Also make sure we depend on the right python3.x
package.
This might help a bit with subtle failures when people install a package from
the wrong distro (https://github.com/matrix-org/synapse/issues/4431).
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...
