`to_json` of Series with period dtype results in AttributeError
#### Code Sample, a copy-pastable example if possible

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

Leads to:

```
~/miniconda3/envs/qgrid/lib/python3.7/site-packages/pandas/core/generic.py in to_json(self, path_or_buf, orient, date_format, double_precision, force_ascii, date_unit, default_handler, lines, compression, index, indent)
   2362             compression=compression,
   2363             index=index,
-> 2364             indent=indent,
   2365         )
   2366

~/miniconda3/envs/qgrid/lib/python3.7/site-packages/pandas/io/json/_json.py in to_json(path_or_buf, obj, orient, date_format, double_precision, force_ascii, date_unit, default_handler, lines, compression, index, indent)
     83         default_handler=default_handler,
     84         index=index,
---> 85         indent=indent,
     86     ).write()
     87

~/miniconda3/envs/qgrid/lib/python3.7/site-packages/pandas/io/json/_json.py in __init__(self, obj, orient, date_format, double_precision, ensure_ascii, date_unit, index, default_handler, indent)
    289             raise ValueError(msg)
    290
--> 291         self.schema = build_table_schema(obj, index=self.index)
    292
    293         # NotImplemented on a column MultiIndex

~/miniconda3/envs/qgrid/lib/python3.7/site-packages/pandas/io/json/_table_schema.py in build_table_schema(data, index, primary_key, version)
    251     if data.ndim > 1:
    252         for column, s in data.items():
--> 253             fields.append(convert_pandas_type_to_json_field(s))
    254     else:
    255         fields.append(convert_pandas_type_to_json_field(data))

~/miniconda3/envs/qgrid/lib/python3.7/site-packages/pandas/io/json/_table_schema.py in convert_pandas_type_to_json_field(arr, dtype)
    115         field["ordered"] = ordered
    116     elif is_period_dtype(arr):
--> 117         field["freq"] = arr.freqstr
    118     elif is_datetime64tz_dtype(arr):
    119         if hasattr(arr, "dt"):

~/miniconda3/envs/qgrid/lib/python3.7/site-packages/pandas/core/generic.py in __getattr__(self, name)
   5272             if self._info_axis._can_hold_identifiers_and_holds_name(name):
   5273                 return self[name]
-> 5274             return object.__getattribute__(self, name)
   5275
   5276     def __setattr__(self, name: str, value) -> None:

AttributeError: 'Series' object has no attribute 'freqstr'
```

This issue also crops up in dataframes where a column is a period Series and `orient='table'`. 

#### Problem description

I can't find anything that says this would be unsupported. 

#### Expected Output

Json string of series representation. 

#### Output of ``pd.show_versions()``

<details>

pd.show_versions()

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.6.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 18.7.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.0.1
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 45.2.0.post20200210
Cython           : None
pytest           : 5.3.5
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.12.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 5.3.5
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None
</details>
