Update tox configuration for tox 4 compatibility (#19)

This commit updates the tox configuration to support running with tox4.
Tox 4, is a major rewrite of the internals of tox and several things
behave quite differently and the configuration format had a large number
of breaking changes. This commit should hopefully enable running tox 3
or tox 4 without issue now.

tox.ini

@@ -1,14 +1,13 @@
 [tox]
 minversion = 2.1
 envlist = py310,py39,py38,py37,py36,neko,lint
-skipsdist = True
 
 [testenv]
 usedevelop = True
 install_command = pip install -U {opts} {packages}
 setenv =
   VIRTUAL_ENV={envdir}
-passenv = QISKIT_NEKO_BACKEND NEKO_TEST_TIMEOUT
+passenv = QISKIT_NEKO_BACKEND,NEKO_TEST_TIMEOUT
 deps =
   -r{toxinidir}/requirements-dev.txt
 commands =