Bump the version of virtualenv in tox.ini to pull in the correct vers…

…ion of pip (#30)

tox.ini

@@ -5,10 +5,11 @@ envlist = lint, py3, pypy3
 # locally installed tox to have a minimum version 3.3.0. This means the names
 # of the configuration options are still according to the tox 3.x syntax.
 minversion = 4.4.3
-# This version of virtualenv will install setuptools version 65.5.0 and pip
-# 22.3. These versions fully support python projects defined only through a
-# pyproject.toml file (PEP-517/PEP-518/PEP-621)
-requires = virtualenv>=20.16.6
+# This version of virtualenv will install setuptools version 68.2.2 and pip
+# 23.3.1. These versions fully support python projects defined only through a
+# pyproject.toml file (PEP-517/PEP-518/PEP-621). This pip version also support
+# the proper version resolving with (sub-)dependencies defining dev extra's.
+requires = virtualenv>=20.24.6
 
 [testenv]
 extras = dev