fix sdist creation in tox.ini

For some reason tox deletes the sdist straight after building it,
which is spectacularly unhelpful.  So bypass tox's installation
"magic" and build it ourselves.

Author: aspiers

tox.ini

@@ -27,4 +27,5 @@ commands = flake8 setup.py git_explode tests
 addopts = -rsxXf
 
 [testenv:sdist]
-usedevelop = False
+changedir = {toxinidir}
+commands = python setup.py sdist