Enable devel testing job

Start testing pytest with code from pytest master and with
prereleases of other dependencies, allowing us to detect
breakages before upstream releases are made.

Author: ssbarnea

.travis.yml

@@ -1,5 +1,11 @@
 language: python
 jobs:
+  allow_failures:
+    - name: devel
+      python: 3.8
+      dist: xenial
+      env: TOXENV=devel
+
   include:
     - stage: tests
       language: node_js
@@ -37,10 +43,9 @@ jobs:
       sudo: required
       env: TOXENV=py37-ansi2html
 
-    -
+    - name: devel
       python: 3.8
       dist: xenial
-      sudo: required
       env: TOXENV=devel
 
     -