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: I55c0f279bbcaa74763132667ab488396a561d35b

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 minversion = 3.2.0
 envlist = linters,docs,py
-skipdist = True
+skipsdist = True
 
 # Automatic envs (pyXX) will only use the python version appropriate to that
 # env and ignore basepython inherited from [testenv] if we set