CI: pin numpy<1.20
- [x] closes #39541
- [ ] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

great! merging this: https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=53418&view=logs&j=eab14f69-13b6-5db7-daeb-7b778629410b&t=ce687173-08c6-5301-838d-71b2dda24510 still failing but can do after @meeseeksdev backport 1.2.x