Drop Python 3.13t support from CI

Python 3.13t was experimental, while Python 3.14t is not. This PR
uses PyCriticalSection_BeginMutex which is only available in Python
3.14+, making Python 3.13t incompatible with the changes.

Removed all Python 3.13t CI jobs:
- ubuntu-latest, 3.13t (standard-large matrix)
- macos-15-intel, 3.13t (standard-large matrix)
- windows-latest, 3.13t (standard-large matrix)
- manylinux job testing 3.13t

This aligns with the decision to drop Python 3.13t support as
discussed in PR #5971.

.github/workflows/ci.yml

@@ -83,9 +83,6 @@ jobs:
           - runs-on: ubuntu-latest
             python-version: '3.12'
             cmake-args: -DPYBIND11_TEST_SMART_HOLDER=ON -DPYBIND11_SIMPLE_GIL_MANAGEMENT=ON
-          - runs-on: ubuntu-latest
-            python-version: '3.13t'
-            cmake-args: -DCMAKE_CXX_STANDARD=20 -DPYBIND11_DISABLE_HANDLE_TYPE_NAME_DEFAULT_IMPLEMENTATION=ON
           - runs-on: ubuntu-latest
             python-version: '3.14'
             cmake-args: -DCMAKE_CXX_STANDARD=14
@@ -105,9 +102,6 @@ jobs:
           - runs-on: macos-latest
             python-version: '3.12'
             cmake-args: -DCMAKE_CXX_STANDARD=17 -DPYBIND11_DISABLE_HANDLE_TYPE_NAME_DEFAULT_IMPLEMENTATION=ON
-          - runs-on: macos-15-intel
-            python-version: '3.13t'
-            cmake-args: -DCMAKE_CXX_STANDARD=11
           - runs-on: macos-latest
             python-version: '3.14t'
             cmake-args: -DCMAKE_CXX_STANDARD=20
@@ -138,9 +132,6 @@ jobs:
           - runs-on: windows-2022
             python-version: '3.13'
             cmake-args: -DCMAKE_MSVC_RUNTIME_LIBRARY=MultiThreadedDebugDLL
-          - runs-on: windows-latest
-            python-version: '3.13t'
-            cmake-args: -DCMAKE_CXX_STANDARD=17
           - runs-on: windows-latest
             python-version: '3.14'
             cmake-args: -DCMAKE_CXX_STANDARD=20
@@ -239,32 +230,6 @@ jobs:
       run: cmake --build . --target test_cross_module_rtti
 
 
-  manylinux:
-    name: Manylinux on 🐍 3.13t • GIL
-    if: github.event.pull_request.draft == false
-    runs-on: ubuntu-latest
-    timeout-minutes: 40
-    container: quay.io/pypa/musllinux_1_2_x86_64:latest
-    steps:
-      - uses: actions/checkout@v6
-        with:
-          fetch-depth: 0
-
-      - name: Prepare uv's path
-        run: echo "$HOME/.local/bin" >> $GITHUB_PATH
-
-      - name: Install ninja
-        run: uv tool install ninja
-
-      - name: Configure via preset
-        run: cmake --preset venv -DPYBIND11_CREATE_WITH_UV=python3.13t
-
-      - name: Build C++11
-        run: cmake --build --preset venv
-
-      - name: Python tests C++11
-        run: cmake --build --preset testsvenv -t pytest
-
   deadsnakes:
     if: github.event.pull_request.draft == false
     strategy: