Update pytest.yml

now uses release version of MNE for testing, and updates python for testing to 3.9

.github/workflows/pytest.yml

@@ -8,16 +8,16 @@ jobs:
         os: [ubuntu-latest, macos-latest, windows-latest]
     env:
       OS: ${{ matrix.os }}
-      PYTHON: '3.8'
+      PYTHON: '3.9'
     steps:
     - uses: actions/checkout@master
     - name: Setup Python
       uses: actions/setup-python@master
       with:
-        python-version: 3.8
+        python-version: 3.9
     - name: Generate coverage report
       run: |
-        pip install -U https://github.com/mne-tools/mne-python/archive/main.zip
+        pip install mne
         pip install pymatreader
         pip install mne[data]
         pip install nilearn