[CI] Compatibility issue with NumPy 1.19.5+ and latest SHAP package

[hcho3]

#6656 (comment)

The latest shap package appears to break when NumPy 1.19.5 is installed:
https://xgboost-ci.net/blue/organizations/jenkins/xgboost/detail/PR-6656/4/pipeline/221

     try:
>       from .. import _cext
E    ImportError: numpy.core.multiarray failed to import

For now, I'm pinning the NumPy version in our CI to 1.19.5, but this is not a permanent fix.

Note. Using shap from conda-forge also fixes the issue, but unfortunately conda-forge has an outdated version of shap.

[Zethson]

Also starts to appear with other major packages. Even Pandas 3.8 requires numpy > 1.20.0

https://github.com/pandas-dev/pandas/blob/master/pyproject.toml#L17

I would love to see numpy 1.20 be supported.

[trivialfis]

Could you please help check the SHAP package? I don't have a computer at hand this weekend.