Add supported interpreters to tox.ini

[Jamim]

Hello,

PyYAML declares support for a wide range of Python versions, but not all of them were included in the envlist at tox.ini, so I'd like to propose changes that:

• add supported interpreters to tox.ini
• 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

With these changes, tox's outcome on my system looks like this:

  py313: OK (30.51=setup[21.49]+cmd[9.02] seconds)
  py312: OK (21.70=setup[11.32]+cmd[10.38] seconds)
  py311: OK (21.23=setup[11.13]+cmd[10.10] seconds)
  py310: OK (31.03=setup[17.04]+cmd[13.99] seconds)
  py39: OK (26.91=setup[13.03]+cmd[13.88] seconds)
  py38: OK (28.26=setup[12.68]+cmd[15.58] seconds)
  pypy310: OK (48.78=setup[24.57]+cmd[24.22] seconds)
  pypy39: OK (44.77=setup[18.24]+cmd[26.52] seconds)
  pypy38: SKIP (0.34 seconds)
  congratulations :) (253.70 seconds)

Best regards!

[Jamim]

Hello @nitzmahone,
While you are here, would you mind reviewing this PR?
Thanks in advance!