Add a CI run with PyPy

[mattip]

There are currently a handful of open issues with "pypy" in them. Would there be any interest in adding a PyPy CI job? Unfortunately the most modern PyPy right now only supports python3.6 which is slated for deprecation by the Python Scientific Data stack at the end of June 2020.

If this is deemed worthy, I see you use both travis and github actions for CI. Travis seems to lag a bit in PyPy support (if their documentation is up to date), while I think github actions is based on Azure Pipelines, which does have latest PyPy installed on the Ubuntu 18.04 image (at the bottom of that link). In any case, NumPy's CI unpacks a PyPy tarball release. So there are many ways to set this up.

[mattip]

xref dask/community#66 where the discussion of PyPy started