Rearrage order of python versions.

Hopefully travisci runs versions in my specified order.

Run non-dev current versions first. Latest ResourceWarning failures
only show up in 3.8 and 3.9, so let them run during the first batch of
4 jobs.

Then backfill with earlier versions and nightly versions.

Note travis has 3.9-dev even though it's tracking a released version.

Author: rouilj

.travis.yml

@@ -6,13 +6,14 @@ cache: pip
 
 python:
   - 2.7
-  - 3.4
-  - 3.6
-  - 3.7
-  - 3.8
   - 3.9-dev
+  - 3.8
+  - 3.7
+  - 3.6
+  - 3.4
   - nightly
 
+
 #I would like to build and test the maint-1.6 and trunk/default
 #but we need different environments for these:
 #  maint-1.6 only python 2, install only psycopg2 version with support for