Allow Python 3.9 when the underlying PySpark is 3.1 and above.

[ueshin]

PySpark 3.1 and above now supports Python 3.9.
We should add the CI and update the upperbound for Python.

[codecov-commenter]

Codecov Report

Merging #2167 (a0f1a54) into master (46c80e6) will decrease coverage by 3.74%.
The diff coverage is 50.00%.

❗ Current head a0f1a54 differs from pull request most recent head ac73d31. Consider uploading reports for the commit ac73d31 to get more accurate results

@@            Coverage Diff             @@
##           master    #2167      +/-   ##
==========================================
- Coverage   95.36%   91.61%   -3.75%     
==========================================
  Files          60       60              
  Lines       13707    13594     -113     
==========================================
- Hits        13071    12454     -617     
- Misses        636     1140     +504     

Impacted Files	Coverage Δ
databricks/koalas/__init__.py	73.17% <50.00%> (-17.86%)	⬇️
databricks/koalas/plot/plotly.py	15.78% <0.00%> (-81.06%)	⬇️
...bricks/koalas/tests/plot/test_frame_plot_plotly.py	23.33% <0.00%> (-76.67%)	⬇️
...ricks/koalas/tests/plot/test_series_plot_plotly.py	26.92% <0.00%> (-71.26%)	⬇️
databricks/koalas/missing/__init__.py	84.61% <0.00%> (-15.39%)	⬇️
databricks/koalas/typedef/typehints.py	82.14% <0.00%> (-13.27%)	⬇️
databricks/conftest.py	92.72% <0.00%> (-7.28%)	⬇️
databricks/koalas/accessors.py	85.13% <0.00%> (-6.76%)	⬇️
databricks/koalas/tests/indexes/test_base.py	94.98% <0.00%> (-5.02%)	⬇️
... and 28 more

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[xinrong-meng]

LGTM, thanks!

[ueshin]

Thanks! merging.