Add datetime as the equivalent python type to TimestampType

[harupy]

Resolves #951

[harupy]

Do I need to write a test?

[itholic]

@harupy i'm not sure but maybe it should be better in terms of usecase example? 😸
(But dont need to write the tests until ueshin or hyukjin comment about it)

[codecov-io]

Codecov Report

Merging #957 into master will decrease coverage by 1.44%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #957      +/-   ##
==========================================
- Coverage   94.52%   93.07%   -1.45%     
==========================================
  Files          34       34              
  Lines        6466     6487      +21     
==========================================
- Hits         6112     6038      -74     
- Misses        354      449      +95

Impacted Files	Coverage Δ
databricks/koalas/typedef.py	83.75% <ø> (ø)	⬆️
databricks/koalas/usage_logging/__init__.py	24.54% <0%> (-72.73%)	⬇️
databricks/koalas/usage_logging/usage_logger.py	50% <0%> (-50%)	⬇️
databricks/koalas/__init__.py	78.04% <0%> (-7.32%)	⬇️
databricks/conftest.py	93.61% <0%> (-4.26%)	⬇️
databricks/koalas/missing/frame.py	100% <0%> (ø)	⬆️
databricks/koalas/missing/indexes.py	100% <0%> (ø)	⬆️
databricks/koalas/plot.py	94.28% <0%> (ø)	⬆️
databricks/koalas/missing/series.py	100% <0%> (ø)	⬆️
databricks/koalas/frame.py	96.03% <0%> (ø)	⬆️
... and 4 more

---

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[harupy]

@itholic Thanks for the comment. I'll wait for them.

[HyukjinKwon]

Yea, let's add a simple test. It's a good practice to add a test whenever it's possible anyway :-).

[harupy]

@HyukjinKwon Ok, I'll add a test.

[harupy]

I'm thinking of adding the following code to test_map in test_series.py (link).

def tomorrow(date) -> datetime:
    return date + timedelta(days=1)

test_df = ks.DataFrame([datetime(2019, 10, 24)], columns=['date'])
test_df['date'] = test_df['date'].map(tomorrow)

[HyukjinKwon]

Sounds good!

[softagram-bot]

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[harupy]

@HyukjinKwon test passed

[HyukjinKwon]

Thanks, @harupy