_metadata items of subclassed pd.Series are not propagated into corresponding SubclassedDataFrame
#### Code Sample

```python
import pandas as pd

# Define a subclass for pd.Series with metadata myprop
class SubclassedSeries(pd.Series):

    _metadata = ['myprop']

    @property
    def _constructor(self):
        return SubclassedSeries

    @property
    def _constructor_expanddim(self):
        return SubclassedDataFrame

# Define a subclass for pd.DataFrame that slices to SubclassedSeries
class SubclassedDataFrame(pd.DataFrame):
    @property
    def _constructor(self):
        return SubclassedDataFrame

    @property
    def _constructor_sliced(self):
        return SubclassedSeries

# make an instance of SubclassedSeries and set myprop
sr = SubclassedSeries([1,2,3])
sr.myprop = 'foo'
print("myprob is", sr.myprop) # Works

# put the SubclassedSeries object into a SubclassedDataFrame and try to get myprop
df = SubclassedDataFrame({'a': sr})
print(type(df.a)) # Works (prints <class '__main__.SubclassedSeries'>)
print("myprob is", df.a.myprop) # does not work (AttributeError: 'SubclassedSeries' object has no attribute 'myprop')
```
#### Problem description

`_metadata` items of `pd.Series` subclasses are not propagated when the `SubclassedSeries` object is put into a `SubclassedDataFrame`. I would expect `myprop` to be available in the new `SubclassedDataFrame`.

#### Expected Output
```
myprob is foo
<class '__main__.SubclassedSeries'>
myprob is foo
```
#### Output of ``pd.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.8.1.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.3.0-42-lowlatency
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : de_DE.UTF-8
LOCALE           : de_DE.UTF-8

pandas           : 1.0.2
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 46.0.0.post20200309
Cython           : 0.29.15
pytest           : 5.4.1
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.1
IPython          : 7.13.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.1.3
numexpr          : 2.7.1
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 5.4.1
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : None
tables           : 3.6.1
tabulate         : None
xarray           : 0.15.0
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : None
numba            : None
</details>
