Pandas install includes hefty 29MB `tests/` directory

[ciarancourtney]

This is particularly relevant to people trying to minimize virtualenvs for AWS Lambda, where a max file size is imposed.

pandas 0.22.0

[TomAugspurger]

Dupe of #19320

Hoping to fix this for 0.23. If you're interested you can take over my PR in #19535

[TomAugspurger]

Where also working on stripping unused symbols from the wheels, which should help even more: MacPython/pandas-wheels#25