Do not install NumPy 1.19.0 on macOS PyPy

[radarhere]

The GHA macOS PyPy job has started failing in master.

It currently uses NumPy 1.19.0 - https://github.com/python-pillow/Pillow/runs/791566962#step:8:107
NumPy 1.19.0 has only just been released - https://pypi.org/project/numpy/#history
It last passed with NumPy 1.18.5 - https://github.com/python-pillow/Pillow/runs/790779493#step:8:107

So this PR disables NumPy 1.19.0 for the macOS PyPy test job.

[radarhere]

For more information, the first error emitted on the failing job is

  File "/Users/runner/hostedtoolcache/PyPy/3.6.9/x64/site-packages/Pillow-7.2.0.dev0-py3.6-macosx-10.7-x86_64.egg/PIL/ImageFilter.py", line 20, in <module>
	import numpy
  File "/Users/runner/hostedtoolcache/PyPy/3.6.9/x64/site-packages/numpy/__init__.py", line 286, in <module>
	raise RuntimeError(msg)
RuntimeError: Polyfit sanity test emitted a warning, most likely due to using a buggy Accelerate backend. If you compiled yourself, see site.cfg.example for information. Otherwise report this to the vendor that provided NumPy.

[hugovk]

Thanks!

Here's a similar NumPy report about it: numpy/numpy#15947

Suggestion is to check site.cfg.example and use OpenBLAS: numpy/numpy#15947 (comment)

If that's easy, let's do it, as I don't think this will go away in future releases.

If not, let's just merge this and report it to NumPy or GHA (at https://github.com/actions/virtual-environments or https://github.com/actions/setup-python) and worry about it later.

[radarhere]

Thanks. I've created #4711 to use OpenBLAS