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: I05c1cc0c2fbf77021cc1e05bc96bee03528c69f0

Author: fungi

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 minversion = 3.2.0
 envlist = py38,pep8
-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
 # ignore_basepython_conflict.