Run py35 without xdist on Travis

Due to the flaky tests in 3.5.0, drop running
py35 with xdist for now in the hope we get better error
messages.

Ref: #5795

Author: nicoddemus

.travis.yml

@@ -42,7 +42,7 @@ jobs:
     - env: TOXENV=pypy3-xdist
       python: 'pypy3'
 
-    - env: TOXENV=py35-xdist
+    - env: TOXENV=py35
       dist: trusty
       python: '3.5.0'