Sort free-threaded Python variants next to vanilla ones

[adamchainz]

Currently, tox-ini-fmt sorts env_list with the t-suffixed variants at the end:

[tox]
env_list =
    py{314, 313, 312, 311, 310, 39, 314t, 313t}

I’d like them to remain in numerical order:

[tox]
env_list =
    py{314, 314t, 313, 313t, 312, 311, 310, 39}

[gaborbernat]

PR welcome 😊