set upper limit to mkl version

MKL version 2024.0.0 that appeared on PyPi on Nov 14, 2023, triggers an as yet
unexplained WinError exception on Windows. This patch limits the dependency to
the most recent version before it, which is 2023.2.1 released Oct 19, 2023,
until these problems are resolved.

.github/workflows/test.yaml

@@ -99,7 +99,7 @@ jobs:
       - name: Configure MKL
         if: ${{ matrix.matrix-backend == 'mkl' }}
         run: |
-          python -um pip install --upgrade --upgrade-strategy eager mkl
+          python -um pip install --upgrade --upgrade-strategy eager 'mkl<2024'
           python -um devtools.gha.configure_mkl
       - name: Test
         run: python -um coverage run -m unittest discover -b -q -t . -s tests

pyproject.toml

@@ -24,7 +24,7 @@ classifiers = [
 [project.optional-dependencies]
 docs = ["Sphinx>=1.8"]
 export_mpl = ["matplotlib>=1.3", "pillow>2.6"]
-matrix_mkl = ["mkl"]
+matrix_mkl = ["mkl<2024"]
 matrix_scipy = ["scipy>=0.13"]
 import_gmsh = ["meshio"]