tox: Remove unsupported Python versions, unnecessary deps

We also bump to tox 4, which handles pyproject.toml files by default
using its own build frontend.

Signed-off-by: Stephen Finucane <[email protected]>

Author: stephenfin

tox.ini

@@ -1,15 +1,11 @@
 [tox]
-envlist = py36,py37,py38,py39,py310,py311,py312,pypy3
-minversion = 1.6
+envlist = py37,py38,py39,py310,py311,py312,pypy3
+minversion = 4.2
 
 [testenv]
 usedevelop = True
-deps =
-  sphinx
-  setuptools>=61
-  setuptools-scm
 extras =
   test
   twisted
 commands =
-  python -W once -m testtools.run testtools.tests.test_suite
+  python -W once -m testtools.run testtools.tests.test_suite {posargs}