Use Python3 for macOS build environment

With the deprecation of Python 2 coming soon, explicitly use
Python 3.

Signed-off-by: Christopher Crone <[email protected]>

script/setup/osx

@@ -36,7 +36,7 @@ if ! [ -x "$(command -v python3)" ]; then
   brew install python3
 fi
 if ! [ -x "$(command -v virtualenv)" ]; then
-  pip install virtualenv==16.2.0
+  pip3 install virtualenv==16.2.0
 fi
 
 #