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: I7880955f656cb171395a62689678c263829bee8b
(cherry picked from commit 56adfba)
(cherry picked from commit 1d0b42f)

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 minversion = 3.2.0
 envlist = linters,docs,py38
-skipdist = True
+skipsdist = True
 ignore_basepython_conflict = True
 
 [testenv]