Revert "ci: Travis: remove pypy3 job for now"

This reverts commit 47022b36cb.

.travis.yml

@@ -34,6 +34,10 @@ jobs:
     - env: TOXENV=py37
       python: '3.7'
 
+    # Coverage tracking is slow with pypy, skip it.
+    - env: TOXENV=pypy3-xdist
+      python: 'pypy3'
+
     - env: TOXENV=py35-xdist
       python: '3.5'
 
@@ -83,6 +87,10 @@ matrix:
   allow_failures:
     - python: '3.8-dev'
       env: TOXENV=py38-xdist
+    # failing due to ResourceWarning exceptions:
+    # https://github.com/pytest-dev/pytest/issues/5317
+    - env: TOXENV=pypy3-xdist
+      python: 'pypy3'
 
 before_script:
   - |