Support pd.DataFrame and pd.Series in return type hints

[HyukjinKwon]

This PR proposes to support the type hints with pandas Series and pandas DataFrame directly.

import pandas as pd
import databricks.koalas as ks
def transform(pdf) -> pd.DataFrame[int]:
    # pdf is actually a pandas DataFrame.
    return pdf + 1

ks.range(10).apply_batch(transform)

     c0
0     1
1     2
2     3
3     4
4     5
5     6
6     7
7     8
8     9
9    10

Note that:

• The type hints with Pandas only works with Python 3.7+.

• The previous way with Koalas DataFrame type hint will still work but will be deprecated once we deprecate Python 3.5 and 3.6.

  import databricks.koalas as ks
  def transform(pdf) -> ks.DataFrame[int]:
      return pdf + 1
  
  ks.range(10).apply_batch(transform)

[codecov-commenter]

Codecov Report

Merging #1577 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1577      +/-   ##
==========================================
+ Coverage   94.55%   94.57%   +0.01%     
==========================================
  Files          38       38              
  Lines        8766     8792      +26     
==========================================
+ Hits         8289     8315      +26     
  Misses        477      477              

Impacted Files	Coverage Δ
databricks/koalas/__init__.py	95.16% <100.00%> (+1.04%)	⬆️
databricks/koalas/frame.py	96.82% <100.00%> (+<0.01%)	⬆️
databricks/koalas/groupby.py	90.53% <100.00%> (+0.01%)	⬆️
databricks/koalas/indexes.py	96.86% <100.00%> (ø)
databricks/koalas/indexing.py	92.25% <100.00%> (ø)
databricks/koalas/internal.py	97.56% <100.00%> (+<0.01%)	⬆️
databricks/koalas/namespace.py	86.09% <100.00%> (ø)
databricks/koalas/series.py	97.63% <100.00%> (+0.01%)	⬆️
databricks/koalas/typedef/string_typehints.py	100.00% <100.00%> (ø)
databricks/koalas/typedef/typehints.py	88.00% <100.00%> (+0.63%)	⬆️

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[ueshin]

LGTM.

[ueshin]

Thanks! merging.