Add supported interpreters to tox.ini

[Jamim]

Hello,

PyYAML declares supported for a wide range of Python versions, but not all of them were included to 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:

  py312: OK (26.83=setup[25.69]+cmd[1.15] seconds)
  py311: OK (13.88=setup[13.04]+cmd[0.85] seconds)
  py310: OK (18.65=setup[17.59]+cmd[1.06] seconds)
  py39: OK (15.34=setup[14.34]+cmd[1.00] seconds)
  py38: OK (14.94=setup[13.83]+cmd[1.11] seconds)
  py37: SKIP (0.02 seconds)
  py36: SKIP (0.01 seconds)
  pypy310: OK (28.65=setup[26.65]+cmd[2.00] seconds)
  pypy39: OK (25.02=setup[22.98]+cmd[2.05] seconds)
  pypy38: SKIP (0.13 seconds)
  pypy37: SKIP (0.01 seconds)
  pypy36: SKIP (0.09 seconds)
  congratulations :) (143.75 seconds)

Best regards!