CI: Install, or upgrade, Python 3 on homebrew.

QuLogic · QuLogic · commit 95f599022c61 · 2020-07-08T18:20:45.000-04:00
It seems it may not be available by default now, so try installing it
first.
diff --git a/.travis.yml b/.travis.yml
@@ -111,7 +111,7 @@ before_install: |
     ci/silence brew update
     brew uninstall numpy gdal postgis
     brew unlink python@2
-    brew upgrade python
+    brew install python || brew upgrade python
     brew install ffmpeg imagemagick mplayer ccache
     hash -r
     which python
