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: I580d3bc78f50c919071ef09f68005399129d3a36

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 minversion = 1.6
 envlist = pep8,validate,docs
-skipdist = True
+skipsdist = True
 
 [testenv]
 basepython = python3