[tests.yml]: Install Cython < 1.0 Before MDAnalysis

MDAnalysis < 3.0 seems to require Cython < 1.0 for compilation (see also
MDAnalysis/mdanalysis#4129 and
MDAnalysis/mdanalysis#3374).

.github/workflows/tests.yml

@@ -68,10 +68,12 @@ jobs:
           restore-keys: |
             ${{ runner.os }}-${{ matrix.python-version }}
             ${{ runner.os }}
-      - name: "Install/Upgrade setuptools, wheel and python-dev-tools"
+      - name: "Install/Upgrade setuptools and wheel"
         # MDAnalysis requires NumPy (>=1.16.0), even for setup.
-        # MDAnalysis < 3.0 requires Cython < 3.0 (see
-        # https://github.com/MDAnalysis/mdanalysis/pull/4129).
+        # MDAnalysis < 3.0 requires Cython < 1.0 (see
+        # https://github.com/MDAnalysis/mdanalysis/pull/4129,
+        # https://github.com/cython/cython/issues/3690 and
+        # https://github.com/MDAnalysis/mdanalysis/issues/3374).
         # Without `python-dev-tools` the installation of MDAnalysis
         # fails on MacOS-latest with Python 3.9 while building the wheel
         # for MDAnalysis.  Strangely, the MDAnalysis wheel is only build
@@ -80,9 +82,9 @@ jobs:
         run: |
           python -m pip install --user --upgrade setuptools wheel
           python -m pip install --user --upgrade python-dev-tools
-          python -m pip install --user --upgrade "Cython < 3.0"
-          python -m pip install --user --upgrade "numpy >=1.16.0"
+          python -m pip install --user "Cython < 1.0"
+          python -m pip install --user "numpy >=1.16.0"
       - name: "Test installation of this project"
         run: |
-          python -m pip install --user --upgrade .
+          python -m pip install --user .
           python -c "import lintf2_ether_ana_postproc"