Add type annotations to window.py

[xinrong-meng]

No description provided.

[codecov-io]

Codecov Report

Merging #1898 (a2254a4) into master (3237002) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1898      +/-   ##
==========================================
- Coverage   94.20%   94.17%   -0.03%     
==========================================
  Files          40       40              
  Lines        9939     9957      +18     
==========================================
+ Hits         9363     9377      +14     
- Misses        576      580       +4     

Impacted Files	Coverage Δ
databricks/koalas/window.py	96.75% <100.00%> (ø)
databricks/koalas/generic.py	93.91% <0.00%> (-1.45%)	⬇️
databricks/koalas/internal.py	96.47% <0.00%> (-0.03%)	⬇️
databricks/koalas/base.py	97.36% <0.00%> (-0.02%)	⬇️
databricks/koalas/frame.py	96.73% <0.00%> (+<0.01%)	⬆️
databricks/koalas/groupby.py	91.47% <0.00%> (+0.28%)	⬆️

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[ueshin]

LGTM.

[ueshin]

Thanks! merging.

[itholic]

Nice