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).
turns out that 0.34.1.1+1 comes before 0.34.1.1+bionic (etc).
The version may only contain "~ 0-9 A-Z a-z + - ." (sorting in that order).
Option 1: replace "+" with something that sorts after +. Options are "-" (but
dpkg-source complains about that) or "." (but that would mean we couldn't
distinguish packaging-only changes from real changes).
Option 2: stick with + and just find something that sorts after 'xenial'. The
only options there are "-", "." (same problems as before), "z", and "+".
Hence, ++1. Sorry.
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...
