CI: pin numpy for CI / Checks github action
failures with numpy 1.20

https://github.com/pandas-dev/pandas/pull/39517/checks?check_run_id=1803471267

xref #39513 and https://github.com/pandas-dev/pandas/pull/38379#discussion_r567753143

conda-forge should pick up 1.19.5 with these changes.
Should probably backport these changes until #39513 and https://github.com/pandas-dev/pandas/pull/38379#discussion_r567753143 are fixed and then revert this pin on the backport branch thanks @simonjayhawkins  @meeseeksdev backport 1.2.x