BUG: Fix to_json when converting Period column
AttributeError  when trying to access frequency string ``freqstr`` directly on Series of ``Period`` type. 

Modified to get it through ``dtype`` of Series, which is known to be ``Period``.

- [X] closes #31917
- [X] fix #31917
- [x] tests added / passed
- [x] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

```python
import pandas as pd
test = pd.Series(pd.period_range('1/1/2011', freq='B', periods=3))
test.to_json(None, orient='table')

>>> '{"schema":{"fields":[{"name":"index","type":"integer"},{"name":"values","type":"datetime","freq":"B"}],"primaryKey":["index"],"pandas_version":"0.20.0"},"data":[{"index":0,"values":{"day":3,"dayofyear":3,"daysinmonth":31,"freqstr":"B","is_leap_year":false,"month":1,"ordinal":10697,"qyear":2011,"start_time":"2011-01-03T00:00:00.000Z","week":1,"weekofyear":1}},{"index":1,"values":{"day":4,"dayofyear":4,"daysinmonth":31,"freqstr":"B","is_leap_year":false,"month":1,"ordinal":10698,"qyear":2011,"start_time":"2011-01-04T00:00:00.000Z","week":1,"weekofyear":1}},{"index":2,"values":{"day":5,"dayofyear":5,"daysinmonth":31,"freqstr":"B","is_leap_year":false,"month":1,"ordinal":10699,"qyear":2011,"start_time":"2011-01-05T00:00:00.000Z","week":1,"weekofyear":1}}]}'
```
Can you add a test for this? @jbrockmendel  Looks like a merge conflict in the whatsnew - can you fix and add a test case in pandas/tests/io/json/test_pandas.py? OK, I'll do it, but first I need to figure it out how to make a test. Have to read more carefully the guide. @colonesej you should just be able to place a test in pandas/tests/io/json/test_pandas.py for this (and can look at other examples there too)

Can you fix up merge conflict here as well? Hello @colonesej! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-06-18 16:09:56 UTC I made this roundtrip test completely based on the one for a time series with a DatetimeIndex. Some points: 

*  I could not find any option to make `read_json` treat the data and generate a PeriodIndex when `orient` equals to `"column" or "index"`. Conversion does not carry frequency information. I'm not sure what exactly should be the behaviour so just converted manually to Period and inferred freq is OK.
* When `orient = "split"`, the test fails as `read_json` cannot convert back the dict created by `to_json`. It works with a DatetimeIndex, which conversion results in float formatted dates. With Period it converts to separated keys for different aspects of Period object. I could not find where these were determined nor what should be preferred.

To illustrate, using `series.to_json(orient='split')` when index is:

DatetimeIndex:

>>> '{"name":null,"index":[946857600000,946944000000,947030400000,947116800000,947203200000,947462400000,947548800000,947635200000,947721600000,947808000000,948067200000,948153600000,948240000000,948326400000,948412800000,948672000000,948758400000,948844800000,948931200000,949017600000,949276800000,949363200000,949449600000,949536000000,949622400000,949881600000,949968000000,950054400000,950140800000,950227200000],"data":[-0.7273489893,0.43763153,0.0064352361,-1.0478282669,0.6605651751,0.2847295244,1.6006677017,-1.2731074841,0.3004426454,0.274394972,0.0402248331,1.5284606656,0.5741635466,-0.3126890855,1.4765392606,0.3663030854,0.3132193099,-0.0730319057,-0.3956868944,1.4626210011,0.2129331654,-0.6671879793,-1.6477379639,0.7626651704,-1.7852083837,0.0707591559,1.3089143648,-1.0282706187,-0.7853299219,-0.2119391625]}'

PeriodIndex:

>>>'{"name":null,"index":[{"day":3,"dayofyear":3,"daysinmonth":31,"freqstr":"B","is_leap_year":true,"month":1,"ordinal":7827,"qyear":2000,"start_time":946857600000,"week":1,"weekofyear":1}, The point with `orient="split"` seems to be connected to how `to_json` converts columns/series with values of `PeriodDtype`. It constructs that dictionary that cannot be converted back by `read_json`. Not directly.

I don't know what should be the behaviour here: 
* change `read_json` to specifically treat when `orient="split"` expecting a dict of attributes, or
* change `to_json` to convert Period values to float (datetime behaviour) or use `__repr__`. The former 
is what other options of `to_json` seems to use.

I made this other test with a frame column with `PeriodDtype` values and eventually test this by the time a solution is proposed. @colonesej can you merge master and try to get green? Yes, but you mean, make the roundtrip tests I've made pass? 

I could have some help about how `to_json` works with `orient = split`.

Or could make a different test, testing based on some expected output template. Will not solve the roundtrip issue but will make it green. What is preferable? If there’s one orient that doesn’t work you can pytest.skip it for now - check some of the other tests in the module for examples @colonesej is this PR still active?