Correct the tox option for skipping sdist generation

The tox option to skip source distribution building is skipsdist,
but this seems to be often misspelled skipdist instead, which gets
silently ignored and so does not take effect. Correct it
everywhere, in hopes that new projects will finally stop copying
this mistake around.

See https://tox.readthedocs.io/en/latest/config.html#conf-skipsdist
and tox-dev/tox#1388 for details.

Change-Id: Id20f1ee2725533e2a06dc6798ead3cc9d70c35eb

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 minversion=3.1.0
 envlist=py37,pep8,bashate,docs
-skipdist=True
+skipsdist=True
 ignore_basepython_conflict=true
 
 [testenv]