Bump to PyPy 3.6

Author: pradyunsg

.travis.yml

@@ -23,9 +23,9 @@ jobs:
     # PyPy
     - stage: secondary
       env: GROUP=1
-      python: pypy3.5-7.0.0
+      python: pypy3.6-7.3.1
     - env: GROUP=2
-      python: pypy3.5-7.0.0
+      python: pypy3.6-7.3.1
 
 before_install: tools/travis/setup.sh
 install: travis_retry tools/travis/install.sh