Add type annotations to Frame (generic.py)

[xinrong-meng]

No description provided.

[codecov-io]

Codecov Report

Merging #1894 (6a43f3c) into master (3237002) will decrease coverage by 0.04%.
The diff coverage is 88.46%.

@@            Coverage Diff             @@
##           master    #1894      +/-   ##
==========================================
- Coverage   94.20%   94.15%   -0.05%     
==========================================
  Files          40       40              
  Lines        9939     9960      +21     
==========================================
+ Hits         9363     9378      +15     
- Misses        576      582       +6     

Impacted Files	Coverage Δ
databricks/koalas/generic.py	93.89% <88.46%> (-1.47%)	⬇️

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[ueshin]

LGTM, pending tests.

[ueshin]

Thanks! merging.