Clean up tox.ini for tox v4

skipsdist now basically means don't install the project at all
(regardless of the usedevelop setting) which creates problems for dib's
entrypoints. Remove skipsdist so that entrypoints can be found. Also, we
remove basepython because this confuses tox v4 on whether or not the
python it wants is present.

Change-Id: I16388a8ad50483228d0b71745f11563f891249c0

tox.ini

@@ -1,12 +1,10 @@
 [tox]
 envlist = pep8,linters,py3
 minversion = 3.18.0
-skipsdist = True
 ignore_basepython_conflict = True
 
 [testenv]
 usedevelop = True
-basepython = python3
 deps=
       -r{toxinidir}/requirements.txt
       -r{toxinidir}/test-requirements.txt
@@ -32,7 +30,6 @@ envdir = {toxworkdir}/venv
 commands = {toxinidir}/tests/run_functests.sh {posargs}
 
 [testenv:cover]
-basepython = python3
 allowlist_externals = find
 setenv =
   PYTHON=coverage run --source diskimage_builder --parallel-mode
@@ -50,7 +47,6 @@ deps=
       -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
       -r{toxinidir}/test-requirements.txt
       -r{toxinidir}/doc/requirements.txt
-basepython = python3
 allowlist_externals =
   rm
 commands =