Update tox.ini

If we remove skipsdist from tox.ini we won't need to specify the runtime dependency for Python 3.6 manually.

Author: frenzymadness

tox.ini

@@ -1,14 +1,13 @@
 [tox]
 envlist = black,mypy,py{36,37,38,39,310,311}
-skipsdist = True
+isolated_build = True
 
 [testenv]
 commands =
     python -m platform
     pytest -v {posargs} test.py
 deps =
     pytest
-    py36: dataclasses  # dataclasses module is in stdlib in 3.7+
 
 [testenv:black]
 deps=black