Run stackless builds in travis before the pypy builds (which take very long),...

Run stackless builds in travis before the pypy builds (which take very long), and require them to work, now that they seem to succeed.

.travis.yml

@@ -80,19 +80,19 @@ matrix:
       language: c
       compiler: clang
       cache: false
-    - python: pypy
-      env: BACKEND=c
-    - python: pypy3
-      env: BACKEND=c
     - env: STACKLESS=true BACKEND=c PY=2
       python: 2.7
     - env: STACKLESS=true BACKEND=c PY=3
       python: 3.6
+    - python: pypy
+      env: BACKEND=c
+    - python: pypy3
+      env: BACKEND=c
   allow_failures:
     - python: pypy
     - python: pypy3
     - python: 3.8-dev
-    - env: STACKLESS=true BACKEND=c PY=2
+    #- env: STACKLESS=true BACKEND=c PY=2
     #- env: STACKLESS=true BACKEND=c PY=3
 
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