Add type annotations for Indexing

[xinrong-meng]

No description provided.

[codecov-io]

Codecov Report

Merging #1884 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1884   +/-   ##
=======================================
  Coverage   94.20%   94.20%           
=======================================
  Files          40       40           
  Lines        9915     9917    +2     
=======================================
+ Hits         9340     9342    +2     
  Misses        575      575           

Impacted Files	Coverage Δ
databricks/koalas/frame.py	96.73% <100.00%> (ø)
databricks/koalas/generic.py	95.35% <100.00%> (ø)
databricks/koalas/indexing.py	92.75% <100.00%> (+0.02%)	⬆️

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[ueshin]

LGTM.

[ueshin]

Thanks! merging.

[xinrong-meng]

@ueshin Thank you!