Add support for API extensions

[scook12]

This should close #1465 and #1285.

[scook12]

Hmm, looks like 3.5 testing is in the pipe? Failing checks 1 and 2 because of f-strings.

From the docs:

If you are using Conda, the Koalas installation and developement enviornment are as follows.

# Python 3.6+ is required
conda create --name koalas-dev-env python=3.6
...

If 3.5+ support is required, I can rollback and add a to fix the docs, as well.

Rest of the failures (3.6+) are from one the doctests of the testing utility that I uncritically replicated from pandas._testing. Will take a look.

Edit:

3.6+ now passing. The testing utility itself is fine, but the outputs in the pipeline don't match up precisely, so I skipped them.
Doctest:

    >>> with assert_produces_warning(False): # doctest: +SKIP
    ...     warnings.warn(RuntimeWarning())
    ...
    Traceback (most recent call last):
        ...
    AssertionError: Caused unexpected warning(s): ['RuntimeWarning'].

Output in the pipeline that caused original check failures is still AssertionError, just more verbose:

    AssertionError: Caused unexpected warning(s): [('RuntimeWarning', RuntimeWarning(), '/home/runner/work/koalas/koalas/databricks/koalas/testing/utils.py', 2)]

Edit 2:

Fixed for 3.5, as well.

[HyukjinKwon]

Thanks @scook12 for working on this. Seems fine from a cursory look but let me take a closer look within few days.

[scook12]

Sounds good! Thanks.

[ueshin]

Otherwise, LGTM. Thanks!

[scook12]

Alright, think I got 'em all. Let me know if there's any other changes you wanted or if I missed something. Thanks!

[ueshin]

Otherwise, LGTM.
I'd leave this to @HyukjinKwon.

Thanks for working on this!

[itholic]

Maybe underline is missing? :)

Parameters
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[ueshin]

nit: indent

[itholic]

Series objects -> DataFrame objects ?

[itholic]

ditto

[itholic]

Index objects -> Series objects ?

[itholic]

Otherwise, LGTM 👍

[scook12]

Thanks for all the code review @ueshin and @itholic! I think (hope) I've killed the last of the typos. :)

[HyukjinKwon]

This is awesome. Thanks @scook12. Merged.

[scook12]

Great! thanks @HyukjinKwon, happy to do it.