travis.yml: install homebrew python for OS X

Our python3 requirements now outstrip those of the build. While we are
at it we can move more of the special casing for Mac into the one
build we have.
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>

.travis.yml

@@ -51,13 +51,6 @@ addons:
       - sparse
       - uuid-dev
       - gcovr
-  homebrew:
-    packages:
-      - ccache
-      - glib
-      - pixman
-      - gnu-sed
-    update: true
 
 
 # The channel name "irc.oftc.net#qemu" is encrypted against qemu/qemu
@@ -239,7 +232,17 @@ matrix:
       os: osx
       osx_image: xcode10.3
       compiler: clang
+      addons:
+        homebrew:
+          packages:
+            - ccache
+            - glib
+            - pixman
+            - gnu-sed
+            - python
+          update: true
       before_script:
+        - brew link --overwrite python
         - export PATH="/usr/local/opt/ccache/libexec:$PATH"
         - if command -v ccache ; then ccache --zero-stats ; fi
         - mkdir -p ${BUILD_DIR} && cd ${BUILD_DIR}