Add support for Debian 11

[smortex]

Fix the following error when the version is not explicitly set:

Error: /Stage[main]/Python::Install/Package[pip]/ensure: change from 'purged' to 'present' failed: Execution of '/usr/bin/apt-get -q -y -o DPkg::Options::=--force-confold install python-pip' returned 100: Reading package lists...
  Building dependency tree...
  Reading state information...
  Package python-pip is not available, but is referred to by another package.
  This may mean that the package is missing, has been obsoleted, or
  is only available from another source
  However the following packages replace it:
    python3-pip
  
  E: Package 'python-pip' has no installation candidate