Fix tox.ini for Tox v4

A variety of cleanups and configuration updates necessitated by tox
v4, but which should be compatible with v3.

Change-Id: I6bf01a77555738c91c466eff303e8f2ff6967f63

tox.ini

@@ -1,10 +1,8 @@
 [tox]
 envlist = ical,pep8
-minversion = 1.6
-skipsdist = True
+minversion = 3
 
 [testenv]
-install_command = pip install -U {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/test-requirements.txt
 
@@ -12,8 +10,7 @@ deps = -r{toxinidir}/test-requirements.txt
 commands = {posargs}
 
 [testenv:ical]
-basepython = python3
-whitelist_externals =
+allowlist_externals =
     mkdir
     ln
 commands =
@@ -29,8 +26,7 @@ commands =
     ln -sf ./doc/build/html/ output
 
 [testenv:pep8]
-basepython = python3
-whitelist_externals = bash
+allowlist_externals = bash
 commands =
   bash test_tools/flake8wrap.sh {posargs}
   python tools/check_chair.py -y {toxinidir}/meetings