Series.astype(str, skipna=True) vanished in the 1.0 release
#### Code Sample, a copy-pastable example if possible

```python
pd.Series([None, 42]).astype(str, skipna=True)
```
results in `TypeError: astype() got an unexpected keyword argument 'skipna'`

#### Problem description

The provided snippet used to work before the 1.0 release. With the `skipna` argument, the `None` was preserved as a real `np.nan` type, instead of being converted to the string `nan`. The `skipna` argument seems to have gone in the 1.0 release and I can't find any way to restore this behavior without a custom function being passed to `apply`.

#### Expected Output

A series with `[np.nan, '42']`

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

<details>
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.1.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.5.1-arch1-1
machine          : x86_64
processor        : 
byteorder        : little
LC_ALL           : None
LANG             : en_US.utf8
LOCALE           : en_US.UTF-8

pandas           : 1.0.0
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 19.3
setuptools       : 44.0.0
Cython           : None
pytest           : 5.3.5
hypothesis       : None
sphinx           : 2.2.1
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.4.2
html5lib         : 1.0.1
pymysql          : None
psycopg2         : 2.8.4 (dt dec pq3 ext lo64)
jinja2           : 2.11.1
IPython          : 7.12.0
pandas_datareader: 0.8.1
bs4              : 4.8.2
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.4.2
matplotlib       : 3.1.3
numexpr          : 2.7.1
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 5.3.5
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : 1.3.13
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None
</details>
