Remove free-threaded 3.13 (3.13t) from CI matrix

CFFI does not support the free-threaded build of CPython 3.13, breaking
any project that depends transitively on cffi/cryptography. Free-threaded
3.14+ is supported, so 3.14t remains.

Closes #206

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: jaraco

.github/workflows/main.yml

@@ -48,8 +48,6 @@ jobs:
           platform: ubuntu-latest
         - python: "3.13"
           platform: ubuntu-latest
-        - python: "3.13t"
-          platform: ubuntu-latest
         - python: "3.15"
           platform: ubuntu-latest
         - python: pypy3.11