Add urllib3-2 environments to tox configuration

Left them on their own line for stylistic reasons, just because putting
all of the py38-py312 permutations on one line was *very* long.

Author: dgw

Diff for: tox.ini

@@ -4,7 +4,7 @@ envlist =
   cov-clean,
   lint,
   {py38,py39,py310,py311,py312}-{requests-urllib3-1,httplib2,urllib3-1,tornado4,boto3,aiohttp,httpx},
-  {py310,py311,py312}-{requests-urllib3-2,urllib3-2},
+  {py38,py39,py310,py311,py312}-{requests-urllib3-2,urllib3-2},
   {pypy3}-{requests-urllib3-1,httplib2,urllib3-1,tornado4,boto3},
   #{py310}-httpx019,
   cov-report