Start testing pypy3
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Thanks for taking a look, our preference is to run more of our CI on Azure compared to Travis. Is it easy enough to move over? (I would wait for some other maintainers to validate this before making any changes)

Ref: Email from @bashtage this week. I'll confess I'm unsure how to provide a PR for the azure bits.... @jcpunk might help to look at the yaml docs for pipelines:

https://docs.microsoft.com/en-us/azure/devops/pipelines/yaml-schema?view=azure-devops&tabs=schema%2Cparameter-schema now that I've chased down the azure bits and gotten the manylinux wheels somewhat sorted, looks like the manylinux containers aren't ready for pypy yet.

I'll close this out for now and revisit when the manylinux container has pypy support.  No sense in hacking it in now. Manylinux support pypy3.7 and pypy3.8. They are installed in manylinux2014:latest (from 5 days ago). For instance, pypy3.8 is in /opt/python/pp38-pypy38_pp73