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`
I don't really follow the issue / fix here: Did the changes go like this?

- numpy 1.17.4: `np.random.BitGenerator`
- numpy 1.17.5: `np.random.bit_generator.BitGenerator`
- numpy 1.18: `np.random.BitGenerator`

Or am I misunderstanding? > * numpy 1.17.4: `np.random.BitGenerator`

For 1.17.4

I see `np.random.bit_generator.BitGenerator` ?

 -> hence I thought the easiest fix is just to require numpy 1.18 ?

Or am i missing something?

Ill also will need to update the test changes made in https://github.com/pandas-dev/pandas/pull/32510/files

@TomAugspurger - mind doing the azp run command -> I don't have permissions. @alimcmaster1 restarted the failing job on travis (was a network error i think). but this looks good to go.