Tests(tox): Change syntax for envlist

`[testenv:]` doesn't allow lists like `[testenv:py27,py3{4,5}]`;
it only allows syntax like `[testenv:py{27,34,35}]`.
This commit changes syntax for `envlist` to make it closer to `[testenv:]`.

Author: phdru

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.15
-envlist = py27,py3{4,5,6,7,8,9,10,11,12},pypy,pypy3,py{27,37,312}-flake8
+envlist = py{27,34,35,36,37,38,39,310,311,312},pypy,pypy3,py{27,37,312}-flake8
 
 # Base test environment settings
 [testenv]