Fix tox4 and setuptools errors

- Remove skipsdist that it was never supported and causes breakage
when used with usedevelop.
- disable setuptools autodiscovery

Change-Id: Icd107c69be1685af9eddf9cc9fe2236d5eb57c5a

setup.py

@@ -17,4 +17,5 @@ import setuptools
 
 setuptools.setup(
     setup_requires=['pbr>=2.0.0'],
+    py_modules=[],
     pbr=True)

tox.ini

@@ -1,6 +1,5 @@
 [tox]
 minversion = 3.18.0
-skipsdist = True
 envlist = py3,pep8,functional-py38
 ignore_basepython_conflict=true