Use Python 3.6 for Mac build (#2764)

* Use Python 3.8 for Mac build

* Another attempt at getting the Mac build to use Python 3

* Fixed syntax error

* More attempts at getting Python 3 on Mac

* More attempts at getting Python 3 on Mac

* More attempts at getting Python 3 on Mac

* More attempts at getting Python 3 on Mac

.travis.yml

@@ -128,8 +128,12 @@ before_install:
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
       brew cask uninstall oclint;
       brew install fftw;
+      brew upgrade python;
       brew install -y https://raw.githubusercontent.com/Homebrew/homebrew-core/5b680fb58fedfb00cd07a7f69f5a621bb9240f3b/Formula/doxygen.rb;
-      sudo pip install -U pytest --ignore-installed six;
+      pip3 install virtualenv;
+      virtualenv -p python3 ~/venv;
+      source ~/venv/bin/activate;
+      sudo pip install -U pytest numpy --ignore-installed six;
     fi
   # The cmake version installed by apt on ARM and PPC is very old,
   # so download a newer version.