Update book.yml

Author: pnavaro

.github/workflows/book.yml

@@ -10,19 +10,19 @@ jobs:
     runs-on: macos-latest
     steps:
     - name: Install brew dependencies
-      run: brew install fftw openblas swig ffmpeg
+      run: brew install fftw openblas swig ffmpeg python@3.8
 
     - uses: actions/checkout@v2
 
     - name: Install python dependencies
       run: |
         cp .pythranrc /Users/runner
-        pip3 install -r requirements.txt
+        /usr/local/opt/python@3.8/bin/pip3.8 install -r requirements.txt
 
     - name: Install Julia packages
       run: julia -e '
           using Pkg; Pkg.add(["REPL", "PyCall"]);
-          ENV["PYTHON"]="/usr/local/bin/python3";
+          ENV["PYTHON"]="/usr/local/opt/python@3.8/bin/python3";
           Pkg.build("PyCall")'
 
     - name: Build the book