.travis.yml: test macos support

.travis.yml

@@ -12,5 +12,17 @@ matrix:
     - name: "python 3.7 MacOs"
       os: osx
       language: shell       # 'language: python' is an error on Travis CI macOS
+      cache:
+        directories:
+          - "$HOME/Library/Caches/Homebrew"
+          - "$HOME/Library/Caches/pip"
+      addons:
+        homebrew:
+          update: true
+          packages: python3
+      before_install:
+        - python3 -m pip install --upgrade virtualenv
+        - virtualenv -p python3 --system-site-packages "$HOME/venv"
+        - source "$HOME/venv/bin/activate"
 install: pip install wheel PyMuPDF pytest pdfrw
 script: pytest test.py