diff --git a/.travis.yml b/.travis.yml
index 59be49011f60..26d548f655de 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -103,7 +103,7 @@ before_install: |
     export PATH=/usr/lib/ccache:$PATH
   else
     ci/silence brew update
-    brew upgrade python
+    brew upgrade python3.6
     brew install ffmpeg imagemagick mplayer ccache
     hash -r
     which python
@@ -124,25 +124,16 @@ install:
   - |
     # Install dependencies from PyPI.
     python -mpip install --upgrade $PRE -r requirements/testing/travis_all.txt $EXTRAREQS $PINNEDVERS
-    # GUI toolkits are pip-installable only for some versions of Python so
-    # don't fail if we can't install them.  Make it easier to check whether the
-    # install was successful by trying to import the toolkit (sometimes, the
+    # Check for the successful installation of GUI toolkits (sometimes, the
     # install appears to be successful but shared libraries cannot be loaded at
     # runtime, so an actual import is a better check).
-    python -mpip install cairocffi pgi &&
-      python -c 'import pgi as gi; gi.require_version("Gtk", "3.0"); from pgi.repository import Gtk' &&
-      echo 'pgi is available' ||
-      echo 'pgi is not available'
-    python -mpip install pyqt5 &&
-      python -c 'import PyQt5.QtCore' &&
-      echo 'PyQt5 is available' ||
-      echo 'PyQt5 is not available'
-    python -mpip install -U \
-      -f https://extras.wxpython.org/wxPython4/extras/linux/gtk3/ubuntu-14.04 \
-      wxPython &&
-      python -c 'import wx' &&
-      echo 'wxPython is available' ||
-      echo 'wxPython is not available'
+    if [[ $TRAVIS_OS_NAME = linux ]]; then
+      # pgi is only installable on Linux.
+      python -mpip install --upgrade $PRE cairocffi pgi
+      python -c 'import pgi as gi; gi.require_version("Gtk", "3.0"); from pgi.repository import Gtk'
+    fi
+    python -c 'import PyQt5.QtCore'
+    python -c 'import wx'
 
   - |
     # Install matplotlib
diff --git a/requirements/testing/travis_all.txt b/requirements/testing/travis_all.txt
index aff3cab60ec6..be1893841204 100644
--- a/requirements/testing/travis_all.txt
+++ b/requirements/testing/travis_all.txt
@@ -1,13 +1,22 @@
 # pip requirements for all the travis builds
 
+--find-links https://extras.wxpython.org/wxPython4/extras/linux/gtk3/ubuntu-14.04
+
 codecov
 coverage
-cycler
-numpy
 pillow
 pyparsing
+
+# GUI toolkits
+
+# pgi is only installable on Linux.
+pyqt5
+# wxpython 4.0.2 and 4.0.3 are broken on OSX.
+wxpython!=4.0.2,!=4.0.3
+
 # pytest-timeout master depends on pytest>=3.6. Testing with pytest 3.4 is
-# still supported; this is tested by the first travis python 3.5 build
+# still supported; this is tested by the first travis python 3.5 build.
+
 pytest>=3.6
 pytest-cov
 pytest-faulthandler