[weblate] Do not install python3-future from Debian

It is now installed with pip. I'll remove the package manually from the
server after deploy.

manifests/weblate/debian_packages.pp

@@ -14,7 +14,6 @@ class tails::weblate::debian_packages () {
     'python3-dateutil',
     'python3-defusedxml',
     'python3-diff-match-patch',
-    'python3-future',
     'python3-jwt',
     'python3-levenshtein',
     'python3-lxml',