ci: Travis: remove pypy3 job for now

Ref: https://github.com/pytest-dev/pytest/issues/5317#issuecomment-499019928

.travis.yml

@@ -37,10 +37,6 @@ 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'
 
@@ -90,10 +86,6 @@ 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:
   - |