CI: Test on Python 3.9 with numpy nightly wheels

Author: effigies

.travis.yml

@@ -62,6 +62,11 @@ jobs:
     - python: 3.8
       env:
         - EXTRA_PIP_FLAGS="$PRE_PIP_FLAGS"
+    # test 3.9 against pre-release builds of numpy
+    - python: 3.9
+      env:
+        - DEPENDS="numpy"
+        - EXTRA_PIP_FLAGS="$PRE_PIP_FLAGS"
     # OSX Python support is basically accidental. Take whatever version we can
     # get and test with full dependencies...
     - os: osx