INSTALL.rst: update Debian dependencies to Python 3

1 job for stable in 8 seconds
Status Name Job ID Coverage
  Test
failed job #2886

00:00:08

 
Name Stage Failure
failed
job Test There is an unknown failure, please try again
From https://gitlab.kwant-project.org/kwant/kwant
x [deleted] (none) -> origin/ci_dependencies
a252ff1..a506cc2 stable -> origin/stable
Checking out a506cc2d as stable...
$ python3 setup.py build
error: Cython-generated file kwant/_system.c is missing.
Install Cython 0.22 or newer so it can be made
or use a source distribution of Kwant.
ERROR: Build failed: exit code 1