Drop packaging from sdist and add check-manifest

MANIFEST.in

@@ -8,3 +8,5 @@ recursive-include test/data bootstrap_cubes *.py
 
 include dev-requirements.txt
 include tox.ini
+prune debian
+prune cubicweb-blog.spec

tox.ini

 [tox]
-envlist = py27, py3, flake8
+envlist = py27, py3, flake8, check-manifest
 
 [testenv]
 deps =
@@ -30,3 +30,11 @@ skip_install = true
 deps =
   flake8
 commands = flake8 --exit-zero --show-source --output-file=code_quality.log {toxinidir}
+
+[testenv:check-manifest]
+skip_install = true
+skipsdist = true
+deps =
+  check-manifest
+commands =
+  {envpython} -m check_manifest