try: fix mac os travis failure

.travis.yml

@@ -37,14 +37,14 @@ jobs:
         - echo 'alias python="python3"' >> /Users/travis/.bash_profile
         - source ~/.bash_profile
         - echo $PATH
-    - name: "Python 3.8.3 on macOS"
+    - name: "Python 3.8.1 on macOS"
       os: osx
-      osx_image: xcode12u
+      osx_image: xcode10.2
       language: shell       # 'language: python' is an error on Travis CI macOS
       before_install:
         - brew install openslide
-        - echo 'export PATH="/usr/local/Frameworks/Python.framework/Versions/Current/bin:$PATH"' >> /Users/travis/.bash_profile
-        - shopt -s expand_aliases
+        - export PATH=/Users/travis/.pyenv/shims:$PATH PYENV_VERSION=3.8.1
+        - travis_wait brew upgrade pyenv && pyenv install $PYENV_VERSION
         - echo 'alias python="python3"' >> /Users/travis/.bash_profile
         - source ~/.bash_profile
         - echo $PATH