BUG: for DataFrame subclass, astype with dict does not keep the subclass
- [x] I have checked that this issue has not already been reported.
- [x] I have confirmed this bug exists on the latest version of pandas.

#### Code Sample, a copy-pastable example

```python
import numpy
import pandas

class CustomDataFrame(pandas.DataFrame):
    @property
    def _constructor(self) -> type:
        return CustomDataFrame

a = CustomDataFrame({'a': [1,2,3], 'b': ['A','B','C']})

print(type(a))

b = a.astype({'a': numpy.int64, 'b': object})

print(type(b))
```

#### Problem description

If I use `astype` on a DataFrame subclass, I would expect that the DataFrame class do not change, and only dtypes of columns change.

#### Expected Output

```
<class '__main__.CustomDataFrame'>
<class '__main__.CustomDataFrame'>
```

But I get:

```
<class '__main__.CustomDataFrame'>
<class 'pandas.core.frame.DataFrame'>
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : f2c8480af2f25efdbd803218b9d87980f416563e
python           : 3.8.5.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.8.0-48-generic
Version          : #54~20.04.1-Ubuntu SMP Sat Mar 20 13:40:25 UTC 2021
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.2.3
numpy            : 1.20.2
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 44.0.0
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
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
