Fix CI mypy command on free-threaded Python

When the version is represented as `3.13t`, the `--python-version`
option needs an operand of `3.13`, not `3.13t`.

(This, and the fix here, will automatically apply to later threaded
Pythons, such as 3.14t, too.)

Author: EliahKagan

.github/workflows/pythonpackage.yml

@@ -97,10 +97,11 @@ jobs:
 
     - name: Check types with mypy
       run: |
-        mypy --python-version=${{ matrix.python-version }}
+        mypy --python-version="${PYTHON_VERSION%t}"
       env:
         MYPY_FORCE_COLOR: "1"
         TERM: "xterm-256color"  # For color: https://github.com/python/mypy/issues/13817
+        PYTHON_VERSION: ${{ matrix.python-version }}
       # With new versions of mypy new issues might arise. This is a problem if there is
       # nobody able to fix them, so we have to ignore errors until that changes.
       continue-on-error: true