str.extract(expand=False) on a string typed series returns an object typed dataframe
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
df = pd.Series(['a1', 'b2', 'c3'], dtype="string").str.extract(r'([ab])(\d)', expand=False)
df.dtypes
```
returns
```
0    object
1    object
dtype: object
```
#### Problem description
the str accessor, when working on string-typed series, should return a string-typed dataframe. It seems that `extract(expand=False)` returns an object-typed dataframe

see @TomAugspurger in https://github.com/pandas-dev/pandas/issues/30966#issuecomment-573724531


#### Expected Output
```
0    string
1    string
dtype: object
```

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

<details>


```
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.0.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.0.0-38-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_IL
LOCALE           : en_IL.UTF-8

pandas           : 1.0.0rc0
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 18.1
setuptools       : 40.8.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
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           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None
```
</details>
