Revert "use a newer pypy3 (https://github.com/travis-ci/travis-ci/issues/6277)"
Revert "use a newer pypy3 (https://github.com/travis-ci/travis-ci/issues/6277)"

This reverts commit 944d1c0a4ab817a9f0dcd124bbf8456520367e8b.

MongoEngine/mongoengine,Davidrjx/mongoengine,cogniteev/mongoengine,Davidrjx/mongoengine,cogniteev/mongoengine