CI: Numpy compat
- [x] closes #34306

Likely related to: https://github.com/pandas-dev/pandas/pull/32510.

Seems doc builds have started using: 
numpy                     1.17.5           py38h95a1406_0    conda-forge
https://github.com/pandas-dev/pandas/pull/34317/checks?check_run_id=700337761

(Previous pipelines where using 1.18)

In numpy 1.17.5 BitGenerator path is:
`np.random.bit_generator.BitGenerator`

In numpy 1.18.0 path is
`np.random.BitGenerator`