Add supported interpreters to tox.ini

PyYAML declares supported for a wide range of Python versions,
but not all of them were included to the envlist at tox.ini.

These changes also:
  * enable the skip_missing_interpreters option
  * add {posargs} to the test command in order to provide flexibility,
    so you can pass arguments to pytest, e.g.: tox -- -k test_dump

tox.ini

@@ -1,9 +1,9 @@
 [tox]
-envlist = py38,py39,py310,py311,py312
+envlist = py3{12,11,10,9,8,7,6},pypy3{10,9,8,7,6}
+skip_missing_interpreters = true
 
 [testenv]
 deps =
     pytest
 commands =
-    pytest
-
+    pytest {posargs}