Travis to use Python 3.7-dev for a little longer

3.7 produces a 403 forbidden error: https://travis-ci.org/simonw/datasette/jobs/450716231#L6

.travis.yml

@@ -3,7 +3,7 @@ language: python
 # 3.6 is listed first so it gets used for the later build stages
 python:
   - "3.6"
-  - "3.7"
+  - "3.7-dev"
   - "3.5"
 
 # Executed for 3.5 AND 3.5 as the first "test" stage: