Now that spice-gtk is in Gitlab CI is run but using Fedora 28 is
failing during configure phase with:

  checking for PROTOCOL... yes
  checking python3 module: six... yes
  checking python3 module: pyparsing... no
  checking python2 module: six... no
  checking python2 module: pyparsing... yes
  configure: error: Python modules six and pyparsing are required
  configure: error: ./configure failed for spice-common
  ERROR: Job failed: exit code 1

Install Python 3 packages needed.

Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Acked-by: Victor Toso <victortoso@redhat.com>