Remove the upperbound for numpy.

[ueshin]

Removed the upperbound for numpy.
numpy>=1.20 only works with pyspark>=3.1, so show a warning when numpy>=1.20 with pyspark<3.1 together.

[codecov-commenter]

Codecov Report

Merging #2166 (3ffc2de) into master (46c80e6) will decrease coverage by 0.01%.
The diff coverage is 50.00%.

❗ Current head 3ffc2de differs from pull request most recent head 873795b. Consider uploading reports for the commit 873795b to get more accurate results

@@            Coverage Diff             @@
##           master    #2166      +/-   ##
==========================================
- Coverage   95.36%   95.34%   -0.02%     
==========================================
  Files          60       60              
  Lines       13707    13711       +4     
==========================================
+ Hits        13071    13073       +2     
- Misses        636      638       +2     

Impacted Files	Coverage Δ
databricks/koalas/__init__.py	89.02% <50.00%> (-2.01%)	⬇️

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[ueshin]

Thanks! merging.

[HyukjinKwon]

@ueshin shall we remove the upperbound in OSS spark too? https://github.com/apache/spark/blob/master/python/setup.py#L272

[ueshin]

Yes, sounds good. I'll submit a PR soon.

[ueshin]

FYI: apache/spark#32908