MAINT: Use Python 3.9 in github action for windows/osx (#1635)

.github/workflows/tests.yml

@@ -127,14 +127,16 @@ jobs:
     runs-on: windows-latest
     strategy:
       matrix:
-        python-version: [3.8]
+        # Using the default python in the Windows 2022 github actions runner
+        # ref: https://github.com/actions/virtual-environments/issues/4856
+        python-version: [3.9]
 
     steps:
     - uses: actions/checkout@v2
-    - name: Set up Python 3.8
+    - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v2
       with:
-        python-version: 3.8
+        python-version: ${{ matrix.python-version }}
     - uses: actions/cache@v2
       with:
         path: ~\AppData\Local\pip\Cache
@@ -155,25 +157,28 @@ jobs:
     runs-on: macos-latest
     strategy:
       matrix:
-        python-version: [3.8]
+        python-version: [3.9]
 
     steps:
     - uses: actions/checkout@v2
-    - name: Set up Python 3.8
+    - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v2
       with:
-        python-version: 3.8
+        python-version: ${{ matrix.python-version }}
     - uses: actions/cache@v2
       with:
         path: ~\AppData\Local\pip\Cache
         key: ${{ runner.os }}-${{ matrix.python-version }}-pip-${{ hashFiles('setup.py') }}
         restore-keys: |
           ${{ runner.os }}-${{ matrix.python-version }}-pip-
+    # We install libmagic to avoid a flaky Fonts error with matplotlib
+    # ref: https://stackoverflow.com/questions/62279920/python-macos-error-unable-to-revert-mtime-library-fonts
     - name: Install Python dependencies
       run: |
         python -m pip install --upgrade pip
         pip install wheel
         pip install --upgrade-strategy eager -e .[sphinx]
+        brew install libmagic
 
     - name: Build the book
       run: |