CI: Install 3.13 explicitly

Workaround-For: actions/runner-images#10758

.github/workflows/maturin.yml

@@ -8,6 +8,8 @@
 # * added --no-index to local package installation during pytest, and remove
 #   MacOS pytest (workaround for 
 #   <https://github.com/PyO3/maturin/issues/1971#issuecomment-2177906051>).
+# * s/3.x/"3.13"/ so that setup-python really installs 3.13, which should be
+#   used then for wheel building as well
 #
 name: CI
 
@@ -46,7 +48,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5
         with:
-          python-version: 3.x
+          python-version: "3.13"
       - name: Build wheels
         uses: PyO3/maturin-action@v1
         with:
@@ -102,7 +104,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5
         with:
-          python-version: 3.x
+          python-version: "3.13"
       - name: Build wheels
         uses: PyO3/maturin-action@v1
         with:
@@ -159,7 +161,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5
         with:
-          python-version: 3.x
+          python-version: "3.13"
           architecture: ${{ matrix.platform.target }}
       - name: Build wheels
         uses: PyO3/maturin-action@v1
@@ -196,7 +198,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5
         with:
-          python-version: 3.x
+          python-version: "3.13"
       - name: Build wheels
         uses: PyO3/maturin-action@v1
         with: