Can't replace tz-aware with None
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
value_a = pd.Timestamp("20130102", tz="US/Eastern")
value_b = pd.Timestamp("20130102", tz="US/Eastern")
df = pd.DataFrame({"A": [value_a], "B": [value_b]})
df.replace(to_replace={value_a: None}).dtypes.drop("A")

```
#### Problem description

Throws `*** TypeError: data type not understood`. On the other hand, this works:
```python
import pandas as pd
value_a = pd.Timestamp("20130102")
value_b = pd.Timestamp("20130102")
df = pd.DataFrame({"A": [value_a], "B": [value_b]})
df.replace(to_replace={value_a: None}).dtypes.drop("A")

```

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

<details>


INSTALLED VERSIONS
------------------
commit           : 7adbc4d0516159cb21fa85c5a476fc67a28af917
python           : 3.7.6.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.15.0-74-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_GB.UTF-8
LOCALE           : en_GB.UTF-8

pandas           : 0.25.0+1928.g7adbc4d05.dirty
numpy            : 1.17.5
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 19.3.1
setuptools       : 45.1.0.post20200119
Cython           : 0.29.14
pytest           : 5.3.4
hypothesis       : 5.3.0
sphinx           : 2.3.1
blosc            : 1.8.3
feather          : None
xlsxwriter       : 1.2.7
lxml.etree       : 4.4.2
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.10.3
IPython          : 7.11.1
pandas_datareader: None
bs4              : 4.8.2
bottleneck       : 1.3.1
fastparquet      : 0.3.2
gcsfs            : None
lxml.etree       : 4.4.2
matplotlib       : 3.1.2
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.1
pandas_gbq       : None
pyarrow          : 0.15.1
pytables         : None
pytest           : 5.3.4
pyxlsb           : None
s3fs             : 0.4.0
scipy            : 1.4.1
sqlalchemy       : 1.3.12
tables           : 3.6.1
tabulate         : 0.8.6
xarray           : 0.14.1
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : 1.2.7
numba            : 0.47.0

</details>
