travis/appveyor: allow failures of the py3.7 build

until they're actually supported by both systems. Neither has a py3.7 image
available yet.

.appveyor.yml

@@ -20,6 +20,11 @@ environment:
     - PYTHON: "C:\\Python36-x64"
     - PYTHON: "C:\\Python37-x64"
 
+matrix:
+  allow_failures:
+    - PYTHON: "C:\\Python37-x64"
+      # appveyor does not yet install python-3.7
+
 install:
   - |
     %PYTHON%\python.exe -m pip install wheel tox

.travis.yml

@@ -28,4 +28,6 @@ after_success:
 matrix:
   allow_failures:
     - python: "3.3"
+      # travis doesn't support py3.7 yet
+    - python: "3.7"
     - python: "nightly"