python-pelican: add missing 'markupsafe' dependency

Needed to be able to run 'make serve' in a pelican directory, else this
happens:

  $ make serve
  cd /home/bfo/pelican-test/output && python -m pelican.server
  /run/current-system/sw/bin/python: No module named markupsafe
  make: *** [serve] Error 1